Hi all,

I've encountered a bug with this example in the docs of getting/setting field parameters of a dataset for use in a derived field. The example works fine for known yt field parameters like bulk_velocity, but if you change every instance of "bulk_velocity" to "my_bulk_velocity" (sample script), then it breaks with this error

It seems as though this breaks because `has_field_parameter` always returns True, and there is no way to know ahead of time what units a user-defined field parameter will take, so the mock field data can't be set up correctly. Nathan suggested via IRC that this might necessitate creating a way to register user-defined field parameters, but it's probably worth brain-storming about if anyone else has any other suggestions!