Making reusable reagent definitions

[jakebeal]

When we are defining experiment plans, we want the reagent definitions to be reusable: it's bad form to end up with lots of copies of arabinose, doxycycline, and DI water kicking around.

We should include an example that shows how to do reagent definitions in a reusable manner, particularly since there are good catalogs for many of them out there, such as PubChem chemical and substance.

There are two basic practices for this:

1. Use ExternallyDefined to include them as Feature objects in sample definitions.
2. Following SEP054, treat them as imports from a non-SBOL package (also dissociated in the case of PubChem), then include them as SubComponent objects

We'll want to show example of PubChem imports, of reusable media recipes (e.g., M9), and of less well-defined substances, such as calibration beads and LB media.