QuantityParamFilterCriterion "unit" field name is confusing

[williamito]

Is your feature request related to a problem? Please describe.
The unit field name is confusing since (1) search appears to actually check the code field of the FHIR Quantity datatype based on the db implementation tests, and the FHIR Quantity datatype also has a different field called unit which search does not act on.

Describe the solution you'd like
Rename the Quantity filter to use code instead of unit to remove confusion.