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.
BTW: This commit currently is showing using your uw.edu email address.  If you'd like this attributed to ara.com you should amend the commit (I can do this for you if you haven't done this before).