USER-DPD: Several updates to *_rx files:

1) Added MY_EPSILON to handle machine precision checks
2) Removed error checks for DPD-RX; enabled use with DPD-E simulations
3) Expanded the EOS functional form to allow corrections
   in the thermo file or on the command line
4) Updated naming convention from fraction to mixWtSite*
5) Changed the name of getParams() method to getMixingWeights()
6) getMixingWeights() now handles fractional and molecular weighting
7) Added optional argument (fractional or molecular) to pair_style command
8) Added argument to specify the exp6 parameter scaling method
   NOTE: Requires additional arguments in the pair coefficients,
   thus command line areguments are NOT backward-compatible.