"Warning: replacing previous import"

[josherrickson]

Came across this from make check while cleaning some of the documentation while addressing another issue. Getting this warning now:

* checking whether package ‘optmatchExperimental’ can be installed ... WARNING
Found the following significant warnings:
  Warning: replacing previous import ‘SparseM::model.response’ by ‘stats::model.response’ when loading ‘optmatchExperimental’
  Warning: replacing previous import ‘SparseM::model.matrix’ by ‘stats::model.matrix’ when loading ‘optmatchExperimental’

I tried manually editing NAMESPACE and DESCRIPTION to ensure stats was before SparseM, but still get the warnings.

This is non-blocking (not causing any issues that I can see), but may need to be addressed when we have more time before we move mlm into optmatch.