Update 3D Euler normal Riemann solver to more modern Fortran.

This is a minor modification of #71.  It is a rewrite by @weslowrie of the 3D Euler normal Riemann solver, using more modern Fortran.  I modified it as follows:

- use `wave` rather than `qwave`, following convention of all other Clawpack solvers
- Squash several separate commits into one, to keep history cleaner