DM-5462 added linearization to IsrTask but had a few loose ends which this ticket aims to correct:
- I intended to enable linearization by default, but somehow lost that change.
- I intended to update obs_test to use null linearization, but I forgot and the previous item meant I didn't catch the omission
- It turns out that the butler data proxy object will not work with functors (attempting to call the retrieved item results in an error, rather than resolving the proxy). This is easily worked around by using immediate=True when retrieving linearizers. This didn't show up until
DM-6356because obs_decam is the only camera that uses linearization lookup tables, and obs_subaru avoids the problem by not returning a proxy.