Field Parameter Registry

[matthewturk]

Having a registry of field parameters would solve three different problems:

• Unknown shapes / sizes of field parameters
• Unknown units ( Field parameters that yt doesn't know about are assumed to be dimensionless #1336 )
• Dynamically defined field parameters

Having a field parameter registry would be a way to specify shape and units at runtime, instead of having to modify the fp_units dict, or adding in hand-checking of shapes and the like in the detection routines.