Downgrade rr columns to lower precision

[rongpu]

This PR makes the following data type changes for the DESI redrock catalog (redrock-*.fits) to reduce the file size. Also see desihub/desispec#2506.

Column	Original Type → New Type
ZERR	float64 → float32
ZWARN	int64 → int32
SUBTYPE	20 characters → 3 characters (or maximum length; see below)
NPIXELS	int64 → int32
NCOEFF	int64 → int16
COEFF	float64 → float32
CHI2	float64 → float32
DELTACHI2	float64 → float32

For SUBTYPE, if any element is more than 3 character long, the column adopts the maximum length; otherwise it is 3 characters in size.

[rongpu]

I have tested this by running redrock on a DESI coadd file, and the resulting catalog is as expected.

[coveralls]

coverage: 42.179% (-0.3%) from 42.509%
when pulling a884542 on downgrade-rr-columns
into 930d936 on main.