Create a 'pure' mode to simulate PURExpress reactions

[murrayrm]

We should create a mechanism to allow modeling of PURExpress reactions in addition to TX-TL reactions. This could be something like the dilution mode described in issue #12. I can think of two approaches to doing this:

• Create an extract type called pure that would be handled by the txtl_extract command and set up a set of reactions corresponding to the PURExpress system.

• Create a new command txtl_pure that creates a tube containing PURExpress. So instead of using txtl_extract to set up a container, we could use txtl_pure and have it set up a different set of reactions.