track volumes across mixes within an experiment, and across experiments (old 53)

[cgevans]

(from @dave-doty)

Track volumes across mixes in one experiment

It would be useful to track volumes across different mixes to ensure that sufficient volumes are available.

First, when using an intermediate mix m1 in downstream mixes, we currently have fairly primitive error-handling. For example, if the volume of m1 is 100 uL, then an error is issued if more than 100 uL of m1 is required in a downstream mix m2. However, suppose m1 is required in two downstream mixes m2 and m3, each requiring 60 uL of m1. No error is given because 60 < 100, even though a total of 120 uL of m1 is required for both m2 and m3.

This could be handled by a new class Experiment, which can track several related mixes used in a single experiment and issue an error if the total volumes don't add up.

[cgevans]

I have a basic implementation of the first thing here, which I'll try to add soon.

[cgevans]

Splitting the other part of this into a new issue.