Fix merging responses in nest-server-mpi

[babsey]

When executing simulation via nest-server-mpi, the result seems not correctly.

In fact, when using a number of processes greater than 1, lets say an example np = 3.
It shows only activity of each 3rd neuron.

It seems that we need to merge the responses of nest-server-mpi.

With this PR, NEST Desktop shows activity of all neurons executed on nest-server-mpi.

[steffengraber]

+1 The logic seems perfectly correct to me. Robustness could be increased by additional checks of values and lists. But only if necessary.

[babsey]

I am working on better solution of combine function.

Thank you for the patience.

[heplesser]

Overall this looks good to me, but I think docstrings for the new methods would be an advantage for long-term maintenance of the code.