Salt2 ColorLaw function correction

[ConstanceGanot]

Hi, an error has been found when Pickling/Unpickling objects than contain sncosmo.models Model objects : it updates the SALT2ColorLaw object coefficients (in salt2utils.pyx) and then crash.

It is due to the function __get_new_args__ that has been made for pickling, and that adds an extra value in the coefficients array to return.
I show an illustration of the bug in the picture below, we begin with some coefficients for the colorlaw and then it add 0s.

I replaced ncoeffs, that is be the number of input coefficients +1, to ncoeffs-1 in the __get_new_args__ function.

[benjaminrose]

Is there a way to add a test, or modify a current test like

sncosmo/sncosmo/tests/test_salt2utils.py

Line 110 in 865f0c9

def test_salt2colorlaw_pickle():

so that we can make sure this bug is not reintroduced?

[ConstanceGanot]

I've modified the test function by adding two 0's at the end of the coefficients list, so that the color law is not modified but the number of coefficients is at the maximum. I tested it on both versions and it raised an error on the previous one.