Morecast 2.0: Add more unit tests for processing grib files

**Describe the task**
Downloading a grib file and then pulling raster bands from it and analyzing their data is a fundamental part of Morecast 2.0 (and Morecast 1). At the moment we don't have any unit tests written regarding handling a grib file - these tests weren't written as part of prior model data PRs because it was tricky getting GDAL to open the grib files and read them as part of a unit test. But it really should be done.

**Acceptance Criteria**
In the context of GFS model data for Morecast 2.0:
- [ ] write 1+ unit tests that assert that 4 raster bands (1 each for temp, RH, U and V wind components) are pulled from a sample grib file when the time interval is 000
- [ ] write 1+ unit tests that assert that 5 raster bands (1 each for temp, RH, U and V wind components, and accumulated precip) are pulled from a sample grib file when the time interval is > 000
- [ ] add any other unit tests as deemed appropriate

**Additional context**
- More unit tests may be necessary for other models (such as NAM)

No files or existing tests are named. Start by locating the Morecast 2.0 GFS model-data processing entry point and current model-data tests, then determine how fixtures open a sample GRIB file. Done means tests cover four bands for time interval 000 and five bands for intervals greater than 000, with other model coverage added if appropriate.

Written by the indexing model from the issue text.