Standard method to specify requirements of each workflow

[mattwthompson]

There should be some standard way of documenting what each workflow needs. We don't want the user to need to install all dependencies at the top-level (including non-Python things like molecular simulation engines) to run a single workflow, so this should be done inside of each workflow. We could

• have a separate requirements.txt in each folder
• maybe something similar but at the high level?
• have a block or two at the top of the notebook that checks all dependencies (including, for example gmx -h or lmp -h) and provides helpful error messages for those that aren't installed, i.e. where each might be found.