Add setup_abfe and analyze_dhdl functionalities

[lmacaya]

Hi. This PR is just minor modifications.

• analysis.py: Label in wplot.png is based on the units given to plot_work_dist and a smoother distribution with a minor step.
• gmx.py: Support for gmx_mpi or gmx installation.
• analyze_dhdl.py: work values for wplot are in agreement with requested units in the flag --units.
• setup_abfe.py: add custom ions and salt concentration.

Looking forward to your feedback. Regards!

LM.