Add switch that optionally finds ESMF from location other than main baselibs

[tclune]

To investigate issues in the main model, it is often convenient to modify/update ESMF independently of the version frozen in the baselibs install.

General approach would be to add an option like ESMF_PATH which defaults to empty, but otherwise FindBaselibs.cmake would use that path to populate the various ESMF cmake variables.

[aoloso]

I will attempt this.