A/C Performance abstract class + graph improvement

[nestorcarmona]

Contains:

• A/C Performance abstract class: modular interface for any A/C perf model
• OpenAP and Poll-Schumann updated code + test fix
• Input graph improvement (networkx) with climb/descent speeds and mach cruise

[fteicht]

Thanks a lot @nestorcarmona for the impressive and great work!
Can you please remove the commented code (not the documentation/explanation comments), and even files which are entirely commented like test_aircraft_perf_model.py? If needed, old code can be retrieved by navigating previous commits. Thanks in advance!

[nestorcarmona]

@fteicht Thank you Florent! Just did some code and file cleanup as requested