Create rpn3_euler_fwave.f90

Riemann solver for 3D Euler equations with f-waves, using Roe-averaged quantities.  Adapted from the current rpn3_euler.f90 solver.  The entropy fix is adapted from the q-wave approach.  For use with dimensional splitting.
I'm not sure what this is for, but if this code is accepted, it will likely outlast the 5.1.X line of development.  Perhaps just delete this line?