Model Class Refactoring

[rdoddanavar]

• Formalize input, output data for models:
  • Set explicit list of input, output variables in model class definition
  • Use references instead of pointers for input variables
• Other refactoring:
  • Rename engine model class to propulsion
  • Formalize lookup table data for model classes:
    • independent numpy array(s)
    • dependent numpy array(s)
• Use proper conventions for python packing and import structure
  • Ex. Instead of import util_yaml use import util.yaml
  • Requires renaming of files
  • Consider using proper __init.py__ files