Optimize operator overloading for XYZ objects

[LSchwiebert]

This pull request creates a more efficient implementation of many of the overloaded operators for the XYZ class. It should run faster and is more readable. It is a cleaner C++ implementation of the XYZ class.

It also changes the XYZ struct to an XYZ class, although making the data members private is complicated by the fact that many other classes access these variables directly. So, they are currently kept as public members.

Also, removed some unused types and UNUSED, which isn't used in the code.

[jpotoff]

Produces matching results for simulations without MultiparticleGas True. Suspect there is another issue that is not related to this patch causing subtle differences in the gas phase multiparticle calculation.