Look into making FuelModels objects use less memory in cases where multiple Behave objects are used simultaneously

[wchatham]

Look into passing a pointer to a FuelModels object to Behave when an instance of Behave is constructed to save memory usage in applications using multiple instances of Behave objects.

[wchatham]

Made FuelModels object static to save memory usage when multiple behave instances are used simultaneously. Not sure if this is the optimal solution, as this may possibly introduce race conditions. Leaving this issue open for now.

[wchatham]

I've added some OpenMP critical sections to methods that alter data in FuelModels. Hopefully this will avoid race conditions associated with FuelModels objects assuming OpenMP is used to parallelize client applications.

[wchatham]

Moved FuelModels outside of Behave, now passing a reference to it at behaves construction. Removed critical sections of FuelModels for now.