Add optional centering of covariates

[svteichman]

• add optional centering of covariates (does not affect the offset term)
• this is built within a control list, which we can later build on with user options
• by default, control$center = FALSE
• runs tests that show that centering does not affect results in most settings and in some settings is less stable compared to not centering
• adds NEWS.md file to describe recent changes to package

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[adw96]

This all looks great. I made a minor change to clarify why one might use center... but really this looks good.

Did we increment the major release earlier and just not add a NEWS then?

Confirming we are not incrementing the version number now?

I'm happy for you to merge when ready

[svteichman]

Yes, I added a NEWS that really refers to the previous release, and did not increment the version here (it was originally because I was thinking centering would be TRUE by default which would be another breaking change so instead of having two major releases back to back just combine them. But now it's only a minor update).

I updated this now to instead update this version as a minor release and separate out these two pieces in the NEWS.md.