API to modify properties of existing connectivity objects

[ntolley]

After discussion with @rythorpe @jasmainak and @carolinafernandezp we've realized that it is currently very difficult to modify properties of the net.connectivity objects after they are made. I'm opening this discussion to work on the potential API for modifying these properties.

I think the most parsimonious solution will be equipping the _Connectivity objects with getter() and setter() functions for the attributes we wish to modify (starting with basic ones right now like connection weight, lamtha, etc.).

Additionally it would be nice to equip them with method that filters the connections to subsets of gids, as well as changes the connection probability. I think these could actually non-interfering properties, where you can specify a set of src_gids and target_gids, and then the probability parameter changes the probability of a connection within that subset.

[jasmainak]

Since _Connectivity already inherits from a dict isn't that already doable?

I was thinking more of a net.update_connection method analogous to net.add_connection