[WIP] auxFields

This adds EquationSystem::m_auxFields as place to store auxiliary fields such as the baseflow fields of the APESolver and the ADRSolver and enables acces to these fields for the filters.

While at it, i noticed that the name of the variable stored in a field is retrieved from the session. I think storing it in the field itself makes more sense and changed ExpList and EquationSystem accordingly. This gives the filters much easier acces to the variable names.

I also found that three methods "EquationSystem::ImportFld" are unused, so instead of adjusting them i just removed them.

Once the variable name change works, i plan to add a callback function to the forcing class to enable access to the forcing field from EquationSystem::m_auxFields, too.

This branch is far from finished but i was hoping to hear your opinions before i continue working on this.