Make it possible to convert between reagents and solvents

[kalekundert]

This came up when I was writing a protocol (~dbp/protocols/cdna/ligate_mrna_linker.py) that basically worked by:

• Creating a big reaction
• Splitting some of the reagents out into a new reaction, with it's own incubation step.
• Splitting the remaining reagents into another reaction, with a new reagent representing the first reaction.

The most natural way to split reagents out of a reaction is to delete all to irrelevant reagents. (In principle the desired reagents could be placed into a new reaction, but this isn't currently supported because reagents hold references to their reaction.) The problem for this application is that:

• It's convenient to have a solvent when setting up the big reaction; e.g. because I can vary volumes without worrying about messing things up.
• But I don't want there to be a solvent when deleting reagents, because the solvent will simply adjust its volume to account to the deleted reagents.

This would be easy to do if "being a solvent" was simply a reagent attribute meaning "derive the volume from the total reaction volume", and was interpreted dynamically by the reaction. I think this is how I originally wanted to write this code, but there were difficulties...

[kalekundert]

This is fixed by the pin_volumes() method added in c6b2b00. I still don't feel like the whole API is quite right, but I'm going to close this issue for now.