Unified framework for volumetrics

[kylegulshen]

This builds off of the work that @joshcombes started in previous branches, but differs somewhat in approach. The CircuitTemplate is an attempt to accommodate some of the variety of circuit families of interest.

The desire is to randomly sample many circuits with various widths and depths from some family. The approach of CircuitTemplate is to (hopefully) enable easy specification of circuit families by composition of basic building blocks. After specifying a family we can generate samples of a given width and depth at will.

The basic building blocks themselves can be quite diverse and require different resources and inputs. This adds to the complexity of CircuitTemplate which must use these disparate blocks in concert.

[kylegulshen]

It would be great to get a review of this. I know it's a bit long, but much of the code is just defining various generators and sequence_transforms which can be tweaked later so long as the shared general design is good. The _main.py file houses the substantive code that would benefit the most from feedback. Everything is auxiliary.