Implement handling of partial bond orders for angles and torsions

[davidlmobley]

I need to finish the job begun in #134 (see #53) and extend the code I added to modulate bond parameters via partial bond orders (when desired) to also modulate angle and torsional parameters.

Our current SMIRFF prototypes (i.e. see #136 ) will not actually modulate angle parameters via partial bond orders, but we should implement handling for that case anyway for completeness.