- Some minor cleanups and simplifications in the Amber prmtop parser.
- Convert coordinates, velocities, and box vectors into Vec3 objects to avoid a
  subtle bug in the units module with respect to silently modifying mutable
  collections in-place when converting to a new set of units.
Merge pull request #536 from swails/fix_inpcrd_units

Convert AmberInpcrdFile data structures from lists to Vec3