Fixed a typo

The linear equation is solved for the correction to the residual, so the 'primes' where missing.
Restructured directory layout, implemented solver

Moved everything related to the pressure solver the the subdirectory /pressuresolver and started coding up a hierarchy of classes for solving the Schur complement system there. Also wrote a first version of the Helmholtz solver in the class helmholtz.Solver.
Ignore output files

Added .vtu and .pvd to ignored files as this contains graphical output which should not be part of the repository.