Fix: Dont read the same files again for every variable

This branch adds a check to prevent EquationSystem from reading the same fld/pts files again for each variable. This gives a huge speedup in some cases.

I also split the EquationSystem::EvaluateFunction method into three new methods: one for expressions, one for fld files and one for pts files to make the code more readable.

However, i think its still a mess with lots of duplicate code and even more things that would make much more sense in some other class. Even worse is that the Forcing base class holds a complete copy of this code. I would like to move the whole Function logic from Forcing and EquationSystem into a new class that lives in SolverUtils in the future. Should we discuss that here or do you want me to write to the mailing list?