fixing inconsistent legendre wavelength basis (#291)

[abhi0395]

Thanks @sbailey for pointing me to the issue of inconsistent legendre wavelength basis definition in code. The issue is detailed in issue#291 have now defined a separate utility function reduced_wavelength() in utils.py, which is called whenever we want to define legendre wavelength basis. It makes the code cleaner and consistent in archetype mode. This consistent definition should also be included in PR#293 and PR#283.

It also fixes a minor case, when an end user does not want to add a prior in the archetype mode. I have implemented a minor logic, that if --archetype-legendre-prior flag is <=0, then no prior will be added in archetype mode.

[coveralls]

coverage: 38.666% (-0.7%) from 39.333%
when pulling cc3c2f4 on correct_legendre
into 2ee8544 on main.

[sbailey]

Thanks. For the record, this PR also updates the wavelength -> [-1,1] mapping to be per-camera instead of using the full wavelength basis across all cameras. I think that is what we want so that the legendre polynomials are orthogonal per camera.

I also added unit tests on reduced_wavelength which caught a wavemax vs. wavemin bug which I corrected.

[abhi0395]

Thank you so much. Luckily, I hadn't run anything big yet.