feat: implement QUANTUM function

[ToddFincannon]

Fixes #87

Implement the QUANTUM function as a C macro with tests taken from the Vensim help documentation.

[chrispcampbell]

Looks good. I pushed a small fix to add parentheses around the macro arguments, which adds a layer of safety for when the macro args are expressions (similar to what we have for other macros).

BTW, I was on the fence about whether this should be a macro or a function; when I worked on reimplementing some things as macros vs functions last year, in cases where an arg appears multiple times on the right side, I sometimes chose to implement it as a C function to avoid redundant calculations. But in the case of QUANTUM, only b appears multiple times and (I'm assuming) it's likely to be a constant value rather than an expression, so I'm less worried about it. And either way, we're getting into micro-optimization territory, so I'm really not too worried about it.