allow the Redrock redshift to be overridden

[moustakas]

For many applications it is useful to be able to override the redshift inferred by Redrock, e.g., to use VI redshifts. This PR adds the hooks necessary to support these redshifts (written in the context of the DESI-2 / tertiary programs).

[coveralls]

Pull Request Test Coverage Report for Build 4677422349

• 5 of 25 (20.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 50.429%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
py/fastspecfit/io.py	2	9	22.22%
py/fastspecfit/fastspecfit.py	3	16	18.75%

Files with Coverage Reduction	New Missed Lines	%
py/fastspecfit/fastspecfit.py	2	13.27%

Totals
Change from base Build 4588352494:	-0.2%
Covered Lines:	2236
Relevant Lines:	4434

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💛 - Coveralls