Multiengine initial capability

[jkirk5]

Summary

Implements compatibility for multiple engine types:

• All components now should support multiple engine types as well as have unittests to demonstrate compatibility
• A new throttle allocation system has been implemented to help with mission convergence
• Misc. cleanup items

Currently multi-engine only works for engine decks, will not work properly with turboprops and known bug with external engines with incoming fix (#280)