WIP dynamic non-state variables

Created by: maedoc

These are elsewhere referred to as auxiliary variables, but here used to accommodate NEST I/O. They are similar to what is already present as variables_of_interest which are used for monitoring arithmetic expressions of state variables, e.g. x1 - x2 for Epileptor LFP, but weren't well documented.

Following also at https://req.thevirtualbrain.org/browse/TVB-2611