src/preproc: preproc_model module --> prop model

[rdoddanavar]

Proposed preproc_model.py that defines parsing routines specific to model input files; ex. aero model, prop model, etc.

Engine model file format: http://www.thrustcurve.org/raspformat.shtml

Module should create data suitable for initializing prop model

[rdoddanavar]

Related to #20

[rdoddanavar]

RASP engine files are *.eng, Rocksim engine files are *.rse (XML)

[rdoddanavar]

import pathlib

print(pathlib.Path('yourPath.example').suffix) # '.example'

Single load function, point towards specific function based on file extension (*.rse vs *.eng)

[rdoddanavar]

Help for working with XML files: https://stackabuse.com/reading-and-writing-xml-files-in-python/