Add support for angle potentials

[1234somesh]

Partially solves #143

Added

• Support to setup angular potentials with pyMBE. All flexible pyMBE templates now support angular potentials: hydrogels, molecules, peptides and residues (including residues with nested residues).
• Sample scripts and tests for the new functionality.
• Template and instance Angle to store information about angular potentials in the pyMBE database.
• Functions define_angle and ``define_default_angle` to define a templates of angular potentials in pyMBE.
• Function create_angle to create instances of angular potential in pyMBE
• Function get_angle_template to retrieve a template of an angle potential in the pyMBE database.

Fixed

• Regression introduced in Replace the pyMBE Pandas dataframe for a canonical pyMBE database #147, wrong handling of hydrogel chains with reverse order of residues.

[pm-blanco]

I have a few final formal revisions to the PR, but overall it looks really good. Thank you very much @1234somesh for an excellent work, I really like how the implementation looks.

[pm-blanco]

LGTM