SED and FILTER conversions for template codes

[sschmidt23]

In discussions of Delight, Sylvie brought up the point that different template-based codes could have different assumed formats for their filter and SED files. I think Delight and BPZ, the two codes in or soon to be in RAIL, use the same conventions, but to be more generic we should include some functions that transform between some common formats, e.g. f_nu vs f_lambda for SEDs.

[sschmidt23]

This issue is only relevant if we add another template fitting code, it's already tagged as back burner, and should definitely be removed from the milestones necessary for a 1.0 release. I'll remove it from LSSTDESC/rail#17 now.