Ensure Connect() does not allow passing parameters that need to be set at the synapse-model level

[heplesser]

Until now, users could pass parameters shared by all synapses of a given type to the syn_spec in Connect(). These parameters were silently ignored as illustrated by the example below, where alpha is not set to 10 either in the connection (which does not have that property) nor in the defaults.

The PR also ports the pertaining tests to Python, in part simplifying and in part extending them.

In [2]: n = nest.Create('iaf_psc_alpha')
In [4]: nest.Connect(n, n, syn_spec={'synapse_model': 'stdp_synapse_hom', 'alpha': 10})
In [5]: c = nest.GetConnections()
In [6]: c.get()
Out[6]: 
{'delay': 1.0,
 'Kplus': 0.0,
 'port': 0,
 'receptor': 0,
 'sizeof': 48,
 'source': 1,
 'synapse_id': 54,
 'synapse_model': 'stdp_synapse_hom',
 'target': 1,
 'target_thread': 0,
 'weight': 1.0}

In [7]: nest.GetDefaults('stdp_synapse_hom')
Out[7]: 
{'alpha': 1.0,
 'delay': 1.0,
 'element_type': 'synapse',
 'has_delay': True,
 'Kplus': 0.0,
 'lambda': 0.01,
 'mu_minus': 1.0,
 'mu_plus': 1.0,
 'num_connections': 1,
 'receptor_type': 0,
 'requires_symmetric': False,
 'sizeof': 48,
 'synapse_model': 'stdp_synapse_hom',
 'synapse_modelid': 54,
 'tau_plus': 20.0,
 'Wmax': 100.0,
 'weight': 1.0,
 'weight_recorder': NodeCollection(<empty>)}

[akorgor]

This PR prevents the scenario that users pass common synapse properties directly to the syn_spec in Connect() and don't notice that these are ignored by throwing an error suggesting to set the respective parameter via SetDefaults on the synpase model, which is an important fix.

The mechanism and the test work like a charm and the code is clear and well-documented.

[nicolossus]

@heplesser Thanks! This looks mostly good to me, I only have a few remarks/questions. See inline comments.

[nicolossus]

LGTM!