Choose ResampleMethod in rf_resample and RasterJoin

[vpipkt]

Allows using a string argument in rf_resample and RasterJoin to choose the resampling method to use.

The choices are (either snake_case or CamelCase would be acceptable):

nearest_neighbor, bilinear, cubic_convolution, cubic_spline, lanczos, average, mode, median, max, min, sum

See GeoTrellis docs for more.

To Do

• [x] Unit test for raster_join
• [x] Fix possible serialization issue in failing tests for DataFrame.toMarkdown
• [x] UPdate release notes
• [x] Update docs chapter about raster join
• [x] update function reference
• [x] ...?

[metasim]

@vpipkt As always, thanks for the fantastic work!