Check that a forecaster's output_variables cover what its temporal downscaler reads #290

A forecaster that drives a temporal downscaler must write every field the downscaler reads, from both the LAM and the ifs-* files. With output_variables (#286) this is up to the user, and a missing field only shows up when the downscaler runs, after the forecaster inference.

The prepare step of the downscaler could check it, but the downscaler reads the forecaster's GRIB through its own input mapping, so the forecaster's names (10u) and the downscaler's input variables don't compare directly. Possible directions: compare the downscaler checkpoint's input variables with the forecaster's written fields through their GRIB params, or check the forecaster's GRIB files for the first init before running the downscaler.

Raised in the review of #286.