Passing in relative experimental results in csv

[hannahbrucemacdonald]

@jchodera pointed out that when we pass in the relative experimental errors, we don't check that they are self-consistent, plus we need a single absolute value such as to correctly offset the absolute plots anyway. ALSO this makes the tracking of experimental errors much more confusing.

The solution is to change the format of the csv file to be something like

### EXP
# lig, exp DG kcal/mol, exp dDG kcal/mol
a, -8.40, 0.22
b, -9.23, 0.33
c, -7.05, 0.22

### CALC
# ligA, ligB, calc DG kcal/mol, calc dDG kcal/mol
a, b, 1.32, 0.65
b, c, 3.45, 0.34

This format would also help with what @dfhahn spoke about with handling multiple simulation methods, we could just chain them together.