Use conditional schemas for domain types to improve error messsages

[letmaik]

This makes a schema change for domain types using a combination of dependentSchemas and if/then. The errors are massively better and I think there's only minor overhead in writing the schema. Once json-schema-org/json-schema-spec#1082 is a thing we can move to that which adds more syntax sugar for this case.

Before:

E           jsonschema.exceptions.ValidationError: {'type': 'Domain', 'domainType': 'Grid', 'axes': {'y': {'values': [1, 2]}, 't': {'values': ['2008-01-01T04:00:00Z', '2008-01-01T05:00:00Z']}}, 'referencing': [{'coordinates': ['t'], 'system': {'type': 'TemporalRS', 'calendar': 'Gregorian'}}, {'coordinates': ['x', 'y'], 'system': {'type': 'GeographicCRS', 'id': 'http://www.opengis.net/def/crs/OGC/1.3/CRS84'}}]} is not valid under any of the given schemas
E
E           Failed validating 'oneOf' in schema:
E               {'$id': '/schemas/domain',
E                'description': 'A Domain, which defines a set of positions and their '  
E                               'extent in one or more referencing systems',
E                'oneOf': [{'description': 'Grid domain: x and y are required, z and t ' 
E                                          'optional',
E                           'properties': {'axes': {'additionalProperties': False,       
E                                                   'properties': {'t': {'$ref': '/schemas/stringAxis'},
E                                                                  'x': {'$ref': '/schemas/numericAxis'},
E                                                                  'y': {'$ref': '/schemas/numericAxis'},
E                                                                  'z': {'$ref': '/schemas/numericAxis'}},
E                                                   'required': ['x', 'y']},
E                                          'domainType': {'const': 'Grid'}},
E                           'required': ['domainType']},
E                          {'description': 'Trajectory domain: mandatory composite '     
E                                          'axis and optional z axis',
E                           'properties': {'axes': {'additionalProperties': False,       
E                                                   'properties': {'composite': {'$ref': 
'/schemas/tupleAxis'},
E                                                                  'z': {'$ref': '/schemas/numericAxis'}},
E                                                   'required': ['composite']},
E                                          'domainType': {'const': 'Trajectory'}},       
E                           'required': ['domainType']},
E                          {'description': 'Default case: domainType omitted, no '       
E                                          'restrictions on axes',
E                           'not': {'required': ['domainType']},
E                           'properties': {'axes': {'patternProperties': {'.+': {'$ref': 
'/schemas/anyAxis'}}}}}],
E                'properties': {'axes': {'description': 'Set of Axis objects, keyed by ' 
E                                                       'axis identifier',
E                                        'type': 'object'},
E                               'domainType': {'type': 'string'},
E                               'referencing': {'items': {'$ref': '/schemas/referenceSystemConnection'},
E                                               'type': 'array'},
E                               'type': {'const': 'Domain'}},
E                'required': ['type', 'axes'],
E                'type': 'object'}
E
E           On instance:
E               {'axes': {'t': {'values': ['2008-01-01T04:00:00Z',
E                                          '2008-01-01T05:00:00Z']},
E                         'y': {'values': [1, 2]}},
E                'domainType': 'Grid',
E                'referencing': [{'coordinates': ['t'],
E                                 'system': {'calendar': 'Gregorian',
E                                            'type': 'TemporalRS'}},
E                                {'coordinates': ['x', 'y'],
E                                 'system': {'id': 'http://www.opengis.net/def/crs/OGC/1.3/CRS84',
E                                            'type': 'GeographicCRS'}}],
E                'type': 'Domain'}

After:

E           jsonschema.exceptions.ValidationError: 'x' is a required property
E
E           Failed validating 'required' in schema['dependentSchemas']['domainType']['allOf'][0]['then']['properties']['axes']:
E               {'additionalProperties': False,
E                'properties': {'t': {'$ref': '/schemas/stringAxis'},
E                               'x': {'$ref': '/schemas/numericAxis'},
E                               'y': {'$ref': '/schemas/numericAxis'},
E                               'z': {'$ref': '/schemas/numericAxis'}},
E                'required': ['x', 'y']}
E
E           On instance['axes']:
E               {'t': {'values': ['2008-01-01T04:00:00Z']},
E                'y': {'values': [1]},
E                'z': {'values': [1, 2, 3]}}

[jonblower]

This looks great! I guess we could consider factoring out the domainType sub-schemas into different files and $ref-ing them, but maybe it won't be necessary.