Make GetStatus and SetStatus of generated models compatible

[jougs]

This was formerly known as https://github.com/nest/nest-simulator/issues/248 and before that as nest.trac.577

GetStatus and SetStatus are complementary functions and it is reasonable to expect that the result of GetStatus can be used as input to SetStatus:

nest.SetStatus(nodes, nest.GetStatus(nodes))
nest.SetDefaults(model, nest.GetDefaults(model))

should work. Currently, it does not work (for almost all models). One reason for this is that GetStatus returns a number of read-only values; when these are sent to SetStatus, they are not read out and an UnaccessedDictionaryItem exception is raised.

Note: SetStatus does not provide a full reset!

old_status = nest.GetStatus(nodes)
... do something ...
nest.SetStatus(nodes, old_status)

will not fully reset the nodes, since SetStatus does not change the contents of buffers for incoming spikes and possibly other variables representing the history of the neuron.

[clinssen]

Related to #363: if set_status() fails at any stage, none of the parameters should be updated. This is achieved in NEST neuron models by making a temporary copy of the parameters struct, updating the copy, and only replacing the actual parameters by the copy at the very end, if everything checks out.

Related NEST simulator issue: https://github.com/nest/nest-simulator/issues/249