Add modified version of Roe's Riemann Solver (RoeM scheme)

Modified version of Roe's FDS to cure carbuncle problem (Ref: J. Comput. Phys. 185(2), 342- 374).
While this scheme has multi-dimensional shock discontinuity sensing term considering its neighborhood pressure, this commit only has one-dimensional shock discontinuity sensing term. 
See note above re abs.  Also compute `((b1*b2)/(b1 - b2))` to avoid necessary divisions.