Improve API documentation

[Fahima-Islam]

Refs #84

List of functions to add API doc or to expose API doc:

• [x] https://github.com/sns-chops/multiphonon/blob/61dd792aa3695281240abf51e9151a308e86163c/multiphonon/redutils.py#L1
• [x] https://github.com/sns-chops/multiphonon/blob/61dd792aa3695281240abf51e9151a308e86163c/multiphonon/backward/singlephonon_sqe2dos.py#L10
• [x] Plotting methods in https://github.com/sns-chops/multiphonon/blob/61dd792aa3695281240abf51e9151a308e86163c/multiphonon/backward/plotutils.py
• [x] https://github.com/sns-chops/multiphonon/blob/61dd792aa3695281240abf51e9151a308e86163c/multiphonon/forward/__init__.py#L7
• [x] https://github.com/sns-chops/multiphonon/blob/61dd792aa3695281240abf51e9151a308e86163c/multiphonon/forward/phonon.py#L21
• [x] https://github.com/sns-chops/multiphonon/blob/61dd792aa3695281240abf51e9151a308e86163c/multiphonon/forward/phonon.py#L30
• [x] https://github.com/sns-chops/multiphonon/blob/61dd792aa3695281240abf51e9151a308e86163c/multiphonon/sqe/__init__.py#L27
• [x] #95