Helmhotz: Brakhage-Werner

[flx-wlf]

We want to wrap the existing C Brakhage-Werner-Routines for the Helmholtz equation

[jdoelz]

This issue can now be solved using Eigen Lazy Evaluation, i.e., by assembling the of the integral operators and then writing

auto M = 0.5M-K+etaV

with a solver similar to the one in the HelmholtzH2 example.