Investigate benefits of ``__slots__`` for core classes

[rileyjmurray]

During my thesis work I wrote Python code that created millions of objects from a particular lightweight class. I was able to speed up the code significantly by using __slots__ for the class in question. I suggest we investigate the possibility of speeding up some of pyGSTi's core classes with this approach.

(I'm just opening this issue as the ideas occurs to me. I'll come back with specific suggestions later.)