Update to rpn3, rpt3, and rptt3_euler.f90

Modified rpt3 and rptt3 for Fortran 90/95 styling.
Fixed a small bug in rpn3_euler.f90, and another small bug in rptt3_euler.f90 (See Issue #74 https://github.com/clawpack/riemann/issues/74)
I know this can be slower but I would favor the removal of a fixed overhead with `maxmrp` and just use the correct amount of memory via `maxm+2*mbc` (which is also not optimal).