Added doc string and parameter_names to DisplacementProbabilityConnector
and CloneConnector classes
Added PerturbedGrid(2/3)D classes (untested)
Added PositionBasedProbabilityConnector for more general point-to-point
connectivity to be used with NineML descriptions
touched up PositionBasedProbabilityConnector documentation and variable
names
Merge remote-tracking branch 'origin/position_connector'