Updating the helical fitting code

[cwalker7]

Description

Starting point for making updates to the helical fitting tool which uses the kHelios fortran package.

Todos

• Added a bunch of input options for get_helical_parameters() so we can input the kHelios path, other kHelios parameters
• Added option for using pdb or dcd structure file rather than the positions stored in cgmodel
• Make everything here work with cgmodel=None
• Error handling for when kHelios is not found/installed
• Update examples folder
• Add tests (at least for the parts which don't run kHelios)

Status

• Ready to go