Improve ring identification in mcfgen.py

[emarinri]

Is your feature request related to a problem? Please describe.

The current ring fragment identification functionality in mcfgen.py uses a custom algorithm to detect rings. This is not optimal, as there are established undirected graph cycle detection algorithms implemented in open source libraries.

Describe the solution you'd like

Utilize networkx to detect rings.

Describe alternatives you've considered

None