Further pynn/sli compatability fixes

* Streamlined identification of common synapse properties in
  Projections. Once a _convergent_connect is called, a list
  self._common_synapse_property_names is available (this is used in
  _set_attributes and shall be used in _get_attributes_xy in the
  future).

* Projection._set_attributes now checks for common synapse properties
  and makes parameters sli compatible prior to invoking pynest.

* native_cell_type now also makes its default parameters pynn
  compatible. The backtransformation (of Sequences to numpy arrays) is
  already handled by _build_params.