Track volumes across experiments

[cgevans]

From #4:

A trickier problem is tracking volumes across different experiments for re-used mixes or strand stocks. This is trickier because, unlike the idea for the Experiment object above, this would require persisting the volume remaining in strand stocks or re-used mixes to disk in a database. The reason this is tricky is that one often re-runs the same code repeatedly to debug, and we would not want that to affect the stored volumes in the database.

One idea would be to use a "functional programming style". Maintain the database as a ordered sequence of files, each file associated to a single experiment. One could introduce a new object called Project, and just like multiple Mix's can be associated to an Experiment, multiple Experiment's can be associated to a Project. The Project would be responsible for maintaining an order on the experiments, and when running code for one experiment, it reads the database file of the previous experiment, say exp7, and writes a new one exp8. If the code is re-run, then this only overwrites the files for exp8.