Consider incorporating ESMF_RegridWeightGenCheck changes that have evolved in CESM #496
Description
CESM - through CIME - has evolved its own version of ESMF_RegridWeightGenCheck: https://github.com/ESMCI/cime/blob/master/tools/mapping/check_maps/src/ESMF_RegridWeightGenCheck.F90 . It looks like this started from the ESMF_RegridWeightGenCheck in the ESMF repo, but then evolved to have some additional features, such as alternative analytical functions that are mapped and pass/fail results. Another difference is that the CIME one is runnable as a separate utility, which seems nice in that you may want to run this checker after-the-fact.
It could be good to bring the ESMF version up to feature-parity with the CIME version so that we can stop supporting this utility in CIME and so that the changes here are available to other ESMF users as well. The pass/fail results printed from the CIME version are particularly helpful, though are also tricky since they I believe they were just determined through experimentation, and are dependent on the source and destination grids... so they are more like rough guides on pass/failure rather than definitive statements.