[gromacs] Reweighting Improvements

[ravenswing]

Current reweight implementation is very slapdash...

• Add 2D functionality.
• Extract auxiliary script to its own module.
• Improve implementation in run_reweight.
• Test reweighting of 1D and 2D FES.
• [EXTRA] Add other methods of reweight (i.e. PLUMED or KDE) to module.
• [EXTRA] Improve code and resolution?