Deprecate get_spaces in favour of direct access to namespaces mapping proxy

[davidesarra]

At the moment get_spaces returns a proxy map of space names and Space instances, which offer two attributes, name and namespace.

As part of a rewrite to make the magics of this package reflect the expected behaviour of the IPython console, the Space class will probably cease to have the current role.

This issue proposes to:

• Deprecate get_spaces raising a warning

• Add a proxy map space names and namespace dicts as jupyter_spaces.spaces

  from jupyter_spaces import namespaces
  
  namespace = namespaces["space_name"]
  namespace["reference_name"]

  The the keys of namespaces will be exactly equivalent toSpace.namespace, whose behaviour is described in the README.

• Remove get_spaces after some time