Add function for calculating circuit_volume

[mstechly]

Is your feature request related to a problem? If so, please describe it.

Circuit volume is a quantity that's often useful and would be good to be able to automatically add it into routines.

How you envision the solution

It would require having a more robust way of accounting for qubits than just looking at the size ports (i.e. a better way of handling local ancillae)

But basically it would calculatecircuit_volume = aggregated_t_gates * qubit_highwater for each subroutine.
We could add it to transform module or once #149 is done, we could add it to postprocessing stages.