New Gas Profile: Free Formula

For the increasing number of weird gas profiles, I added a profile called `gasFreeFormula` that allows the user to define a profile in SI units.

The drawback of this is, that is uses double precision for the position, but since the user has to define a normalized profile *in density* ( `f(x) -> [0,1]` ) it is still ok and avoids heavy coding for such a simple task.