Add support for Parameter objects for indegree and outdegree in syn_specs

[hakonsbm]

Attempts to fix #1615 by implementing the possibility to pass a Parameter object as indegree in fixed_indegree or as outdegree in fixed_outdegree.

Example:

conn_spec = {'rule': 'fixed_indegree', 
             'indegree': nest.random.normal(mean=20.0, std=2.0),
             'p': nest.spatial_distributions.gaussian(nest.spatial.distance, std=0.5),
             'allow_multapses': True}

[nicolossus]

Looks good! I think that one now can pass a Parameter object as indegree/outdegree should be documented. I guess doc/htmldoc/synapses/connection_management.rst would be a good place for the documentation, but there might be another more suitable location.

[nicolossus]

Thanks! I only have a minor grammatical suggestion, otherwise I approve once all tests pass

[jhnnsnk]

Looks good to me! Thank you very much for this useful addition! 👍