Gas model

[eldond]

• Adds a simple model for converting a gas command in V(t) into a flow rate vs. time. Includes slightly nonlinear equilibrium flow(voltage) as well as dead time and first order lag between flow and command.
• The instant gas model that goes from equilibrium flow(voltage) is taken from DIII-D.
• The dynamics model is just FOPDT.
• Model is sensitive to gas species.
• All tuning is made up or copied from DIII-D and not valid for SPARC; these are just sample data that have to be fixed up later.
• Should be adequate for setting up any workflow that needs an actuator model.
• Close #6

[eldond]

Force push was to rebase and squash all the cleanup/fix commits together. The log must be clean.