Make GetStatus and SetStatus compatible

[heplesser]

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.

The same should also apply to getting and setting status on connections.

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.

This is one part of a follow-up of trac.577.

[heplesser]

Solving this would require knowing which entries in status dicts are read-only. This can be solved once all models are generated using NESTML.

[jougs]

Closing here as this is now a NESTML issue. Please go there for updates and discussion.