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.
Please don't do this.  Your contribution is captured in the Git log, including when you last modified the file.  Putting authorship/modification information into the source code itself gives us two places to track things redundantly.