improved testcase for eigenvalue computation in fwave-solver.c

now 2 pairs of eigenvalues will be tested
new tests for flux and eigencoefficients in FWaveSolverTest.hxx
bugfix in fwave-solver.c: calculate_updates

wrong application of the formula:
fixed conditions in lines 89, 94, 101, 106:
'alpha' had to be replaced by 'lambda_roe'