Cerf

**Purpose:**
States rasters were assumed to have a nodata value of 126 which gets filled by 0 upon converting the substation vector data to a raster using the states raster as a template. However, if a valid value were to be 126, it would get nullified. This fix modifies the nodata value in the template states raster to -np.nan to ensure that no real value will be masked when filling with 0.

**Additional:**
- The code was looking for a key that no longer exists when building the _rval_ variable. The key was updated to reflect variable name changes that had happened previously.
- Added raising an error if the min_volt field is missing in the provided substation data

**Notes:**

- New variables available in outputs
- No longer supporting internal unit conversion for fuel price and fuel CO2 content from native GCAM outputs. CERF now expects its native units as described in the docs.
- Performance boost by excluding 0 expansion techs from initial competition
- Add units to variable names for outputs and configuration

Release for JOSS publication

Release prep

See https://immm-sfa.github.io/cerf/release_notes.html for change log.

Change:
- See 68 for change log