add rml:datatypeMap

[VladimirAlexiev]

If we can attach a lang tag to a literal dynamically (see #9), maybe we should be able to do the same for datatypes. This won't happen very often but it may happen that the datatype of some value is stored in a different column.

• rml:datatypeMap is an IRI term map that produces a datatype IRI to be used in the current object.
• rr:datatype is a constant IRI used as fallback when the IRI designated by rml:datatypeMap is null for the current row.

See some YARRML and test data in this comment of #9.

• It's interesting to consider how to add prefixes, eg how to get from a string value like "integer" to a IRI like xsd:integer