Dust law improvements - Githubissues

This is more a to-do/wishlist, following on from #1431. Over the past few months we've made some improvements to the dust laws implemented in SNANA. There are still some things that could be improved further, listed below:

[ ] Re-visit the F99-like dust laws depending on the outcome of https://github.com/karllark/dust_extinction/issues/243.
[x] Make sure all $R_V$ and $\lambda$ limits are enforced properly.
[x] Possibly implement the second derivatives in the F99 function directly. Currently I have hardcoded some matrices to handle this. It was okay when it was just F99, but now there are three F99-like laws with slightly different arrangements of spline knots, things have really gotten out of hand.
[x] Add the option to modify the $f_A$ parameter in the Gordon et al. (2016 dust law. I'll need help from @RickKessler with this. Would also be useful to have input from @djbrout about what your intended use case was? (see also #1434)
[x] Implement additional $R_V$-dependent extinction laws, to "complete the set" of curves from dust_extinction. Outstanding ones are listed below.
[x] Document these things somewhere (done in #1433).

More dust laws to add:

[x] Maíz Apellániz et al. (2014): This uses a spline, but not quite the same type as F99 (as far as I can tell). They give IDL code in the paper, and an implementation is included in dust_extinction. Only optical and NIR (no UV).
[x] Fitzpatrick et al. (2019): The implementation in dust_extinction is based on a B-spline (but that might just be another boundary conditions issue). The paper talks about cubic splines though, and based on past codes Fitzpatrick seems like a natural cubic kinda guy (a man after my own heart). In any case, TBC whether this will be easy or annoying (done in #1437 ).
[x] Goobar (2008): Power law for circumstellar dust.

RickKessler / SNANA

Dust law improvements #1432