Clarify improper definition in spec, including figure illustrating torsions

[jchodera]

The current definition of improper torsion of improper torsions appears to not be fully explicit in specifying exactly which torsions the improper functional form is applied to. A diagram would also help substantially, since the ordering of atoms is not obvious (e.g. atom index 2 is the central atom).

[pavankum]

Slightly off topic but I love @lilyminium 's table of impropers, https://github.com/lilyminium/improper_orders.