implemented computation of eigencoefficients in fwave-solver.c

wrote new function 'eigencoeffis(State ql, State qr, Vector2D roe_eigenvals)' in fwave-solver.c
changed parameters for eigencoeffis() in fwave-solver.c

df will be calculated outside of this function now, as we will need the value again later
implemented calculate_updates in fwave-solver.c

fwave-solver.c now needs stdlib.h to run