change to standard NLL fit objective

[lukasheinrich]

right now we use "twice_nll" as a fit objective and in the test statistic a simple diffence

twice_nll_constrfit - twice_nll_globalfit

but rather we should just to a NLL fit and in the test stat do

2*(nll_constrfit - nll_globalfit)

this will require updating some test reference numbers in the tests

[matthewfeickert]

@lukasheinrich while I'm not against this, what is the primary motivation for the change?

[lukasheinrich]

it's more customary to fit the NLL and I think we might have slight bus re errordef in minuit because we're fitting a scaled objective

[matthewfeickert]

I think we might have slight bus re errordef in minuit because we're fitting a scaled objective

Good enough reason for me. 👍

[alexander-held]

I just came across this again, but the errordef being used is fine, so no bugs there.