Refactor/document for enabling use of GEDI Subsetter outside of MAAP

[chuckwondo]

Code is currently coupled to the maap-py library, allowing use of the GEDI Subsetter only from within MAAP.

We want to refactor the code so that it can be used easily either within MAAP or outside of MAAP, and provide documentation on how someone could then use it outside of MAAP.