Works on PolarLaplacian app

Reduced code duplication by introducing a linear_solve() function that
get's called for both possible cases (geometryMap and pimpedForm). We
can now also select Gauss-Seidel or CG as solver. Adapted parameter
file.
24 jobs for mohr/PolarLaplacian in 110 minutes and 29 seconds (queued for 3 seconds)