Refactoring `generate_Ch_list` function.
Refactoring `generate_Ch_list` function.

Removing duplicate code by filtering out the list of
`chiral_types` in the final generate list of chiralities
using list comprehension.

Also removing the constant `edge_types` and `Ch_types` and
adding the constant `CHIRAL_TYPES` list of chiral type
string names.

androomerrill/scikit-nano,androomerrill/scikit-nano,scikit-nano/scikit-nano,scikit-nano/scikit-nano