generic script for solvent free energy differences "mdpow-pxy".

[orbeckst]

We need a way that the Pcw and Pow calculations can use water runs that were not originally part of the same MDPOW run (not in the same directory).

Perhaps water is too restrictive as a directory name and alternatives need to be allowed. Or have subdirectories with water model name but the first approach is more flexible.

1. Allow multiple solvent directories (eg tip4p, spce, cyclohexane, octanol, ...)
2. New script mdpow-pxy that calculates the difference between solvents x and y.