Support `mmrm`

[vincentarelbundock]

On 2023-01-11 the package did not supply a predict() method with a newdata argument.

[adrianolszewski]

Hello,
Just wanted to make a small update that the mmrm package now provides the predict() function in both CRAN release and the GitHub latest one. If the bandwidth allows - technically this package can be supported now.

[vincentarelbundock]

Thanks for the nudge. I think we are very close to a working implementation here: #1000

I feel that none of the people involved have had time to reach the finish line yet, but I do plan to get to this eventually.