Add the concept of categories to var types

[gordonbrander]

This lets us get lists of variables per category type, and we can get rid of the openag_brain.var_types file, which too easily gets out of sync with this file.

[coveralls]

Coverage decreased (-0.5%) to 83.128% when pulling 648b7d3 on gordonbrander:one_var_to_rule_them_all into d90113e on OpenAgInitiative:master.

[gordonbrander]

This has been tested on Raspberry Pi.

[LeonChambers]

This is actually very confusing. There is now a list of "categories" in a categories.py file which applies to modules and their inputs/outputs and a different list of "categories" in var_types.py which applies to the environmental variables. One of these 2 concepts should be renamed.

[gordonbrander]

That's a good point. I don't care about the name, just need a method to group var_types without resorting to editing in 2 places.

[LeonChambers]

Yea, I definitely think this change is for the better if the naming is switched out. Maybe "groups" could work?

[gordonbrander]

wfm. Will do a followup PR for openag_python and for openag_brain

[gordonbrander]

#36
OpenAgricultureFoundation/openag_brain#133