potential fix for resamplemethod in reproject

[jdries]

https://github.com/locationtech/geotrellis/issues/3541

Overview

Proposed fix for a very important bug in reprojecting to lower resolution. I think it's better (more correct) than previous behaviour, but not sure yet if it is 100% accurate.

It seems to me that the downsampling now happens in the original projection system, and then it is warping to the new CRS. This will work fine if the low resolution pixel is rectangular in both projection systems.

Checklist

• [x] ./CHANGELOG.md updated, if necessary. Link to the issue if closed, otherwise the PR.
• [x] Module Hierarchy updated, if necessary
• [x] docs guides update, if necessary
• [x] New user API has useful Scaladoc strings
• [x] Unit tests added for bug-fix or new feature

Demo

Optional. Screenshots/REPL

Notes

Optional. Ancillary topics, caveats, alternative strategies that didn't work out, anything else.

Closes #XXX

[jdries]

The fix works but I had to fall back to old (bad) behaviour in cases at the edge of valid target crs bounds. My method to compute target cellsize resulted in an error there.