Update CI

[slager]

This updates the VISCfunctions CI to follow the working VISCtemplates CI, addressing #90.

The runners with old R succeed, and the runners with new R fail, which is as expected because the latter are catching the test errors identified in #89.

I also had to move one latex line from the vignette header to the body to get it to build.

[kelliemac]

This looks good to me but I am not super familiar with the details of VISCfunctions so I think I should let @mayerbry be the final approver!

[slager]

Added a codecov GHA workflow as well, since one was in the old GHA workflow.

Not sure why the test-coverage workflow is not working right now. Any ideas @kelliemac? My best guess is that either there's a syntax error in the workflow somewhere, or that it won't work till it is merged to main.

[kelliemac]

@slager I would bet that it doesn't work until it's merged to main. I ran into a similar issue when I created a new github workflow before for VISCtemplates. I wouldn't let this hold up mergning.