created header file for fwave-solver.c

1.) struct definitions (State, Vector2D) were moved to the header
2.) some compiling bugs were also fixed:
  a) definition of the array 'z' in line 73
  b) if-condition in line 61
implemented cxxtest-testsuite for fwave-solver.c

the testsuite is located in FWaveSolverTest.hxx