Added support for mgcv::gaulss for find_formula and find_variables

[hhp94]

Fixes #840

Added support for mgcv::gaulss in find_formula and find_variables.

For R/find_formula.R/find_formula.gam, i'm pretty sure the is.null is supposed to be outside of the if mi$familly == x checks. Other than that, just a straight forward case of checking if the family is "gaulss" and if yes then extract not only the conditional variables for the location parameter but also the variables for the scale parameter.

For R/find_variables.R/find_variables, have to update the choices to also have "scale".

Added tests for this change. Sorry if there are any mistakes, this is my first PR ever

[strengejacke]

Thanks a lot! I turned the if...else into a switch statement (suggested by lintr), and also addressed some other lintr-issues that were outside the scope of this PR (but still flagged, because they appeared in the files that have been changed - lintr checks modified files in PRs, so lintr-issues that existed before but were not spotted are now mentioned).

[strengejacke]

Thanks, I think remaining failing tests are not related to this PR

[hhp94]

The rstats community is great. Merry Christmas! I'll lookingto model_info and the link function related functions soon.