Add support for AMBER, OPLS and CHARMM force fields

[lunamorrow]

In reference to #48 and after discussion with @RangikaM. The PolyConstruct paper review comments highlighted a need for support for the topology (ITP) files of other force fields. New read and write parsers need to be developed off of the existing GROMOS parser to support other GROMACS supported force fields including: AMBER, OPLS and CHARMM.

Checklist of development (for transfer/reference in the PR):

• Reader for AMBER forcefield ITP
• Writer for AMBER forcefield ITP
• Test cases for AMBER read/write
• Reader for OPLS forcefield ITP
• Writer for OPLS forcefield ITP
• Test cases for OPLS read/write
• Reader for CHARMM forcefield ITP
• Writer for CHARMM forcefield ITP
• Test cases for CHARMM read/write

[lunamorrow]

OPLS, CHARMM and AMBER forcefield itp's are now fully supported (see #52), so this issue will be closed.