Consolidate all volume and intersection functions

[mboratko]

I think it would make sense to consolidate all volume (and intersection) functions into a single one, where if the variance is zero we revert to the "hard" versions of these functions.

[dhruvdcoder]

Do we want to unify all intersections and Volumes under single function? Or do we have 'if conditions' inside each one to revert to hard version based on beta value?

[mboratko]

I meant the latter.

Note that this is a situation where the functional interface and class-based interface can (and should) actually be different. For example, the functional interface will include an "if" statement based on the variance (for example), but a class will assign the correct branch during initialization.

(Maybe this is a premature optimization though.)

[dhruvdcoder]

We can leave this open and come back to it when we have a better idea about whether the optimization is needed or not.

[dhruvdcoder]

Add an example in the usage example notebooks.

[trangtran72]

Trang will address this in the upcoming PR