updates to catchment restart regridding

This can be considered parts 2 and 3 of 3 fixes/upgrades to how catchment regridding treats tile sampling.

See comment by @gmao-rreichle in https://github.com/GEOS-ESM/GEOSgcm_GridComp/pull/92#issuecomment-540089789

this fix addresses the most important of three separate problems with regrid. The other two are (i) the spatial mapping of source and target tiles and (ii) consideration of the WEMIN parameter in the regridding of snow depth. These are being addressed.

These parts are:

1. Addressed by @smahanam with #92 . Fix so that soil heat content deliveries consistent soil temperature with regridding (with consideration for ice fraction).
2. updating the spatial mapping of source and target tiles to be more accurate (but a bit slower)
3. considering the model parameter WEMIN, which should be consistent with both input and output restarts. WEMIN can be considered the threshold value that controls the linear ramp between snow water equivalent and snow fraction:
   
   It decreases between Old Land and New Land, and regridding should be able to account for it's affect.

@smahanam is making good progress on (2), and we're firming up our plan for (3). @wmputman has been given a heads up.

Review PR #92 and the linked comment first to understand the completed soil heat content fix and the remaining regridding problems. The work is done when source and target tile mapping is more accurate and WEMIN is accounted for consistently between input and output restarts, but the issue names no files or tests.