pyNN.nest: Further handling of syn params

* ValueError is raised when CommonProperties of synapse type is assigned
  different values in the same Projection (validated by added testcase).

* Shape (1,) arrays of parameters (as produced by the FromListConnector)
  are converted into the corresponding scalar because otherwise
  nest.SetDefaults interprets them as vectors.

* numpy.bool_ are converted to bool because nest.SetDefaults does not
  handle them.