Connection rule for direct support of clusterd connectivity

[heplesser]

This PR introduces experimental support for direct creation of clustered connectivity. It is used as follows:

nest.Connect(a, b, 
             {'rule': 'clustered_fixed_total_number',
              'N': 100,
              'num_clusters': 2},
             nest.CollocatedSynapses({'weight': 100},  # first element is for intra-cluster connections
                                     {'weight': 3}))   # second element is for cross-cluster connections ```

The node collections a and b are split into num_clusters clusters in a round robin fashion each and connectivity created using fixed-total-number for each pair of clusters. The first syn specs in the "collocated" synapse are used for connections from a given cluster in a to the corresponding cluster in b, the second syn specs for connections other than the remaining clusters in b.

Notes:

• If populations do not divide evenly into clusters, the total number of connections may be less than N.
• CollocatedSynapses is abused here for experimental purposes—only single synapses can be created.
• The implementation is based on rather too much code duplication.
• The PR is provided primarily to allow evaluation of the performance gains.