new "asserts" in fwave-solver.c

in the eigenvalue computation, a water column greater than zero will be asserted
changed parameters for eigencoeffis in fwave-solver.c

1.) eigencoeffis will now calculate the eigencoeffitients directly from the quantities ql and qr and the roe_eigenvalues
  a) the parameter list is now (State ql, State qr, Vector2D roe_eigenvalues)
  b) the difference in the flux function is calculated in this function now
  c) the corresponding unit-test was modified accordingly

2.) doxygen documentation was added in fwave-solver.h
initialized doxygen documentation