impoer NEST_SYNAPSE_TYPES from nest.__init__.py

The variable NEST_SYNAPSE_TYPES is defined once and then used by pyNN.nest.standardmodels.synapses._get_nest_synapse_model to check for available synapse types in NEST. Before, the function called nest.Models(mtype='synapses') at every function call, which slowed down things a lot, especially in the case of many single projections.