Support terminal AA CN expansion

[IAlibay]

Fixes #30

Main changes:

• In terminal AA expands C-N to residue-1 instead of residue+1.
• Adds check to make sure that N is bonded to C (in case of chain hopping), if not, will default to above terminal AA case.

[codecov]

Codecov Report

Merging #31 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.