Migrate training sites reduction tool to ArcGIS Pro 3.x + some usability improvements

[em-t]

Issue #159

In this PR:

• Convert from old arcpy conventions to new ones.
• Usability improvements / bug fixes:
  • Clear the selected training points afterwards.
  • Don't remove the selected mask layer from the map. (This was happening due to how a feature layer was being made from the mask layer.)
  • Support more types of mask layer: feature layer, feature class, raster layer and raster dataset. (Previously only feature layer was supported)
  • Support having no mask layer (since it's not really necessary for site reduction).

I've tested a number of different scenarios, and at least these all seem to work and produce sensible outputs:

• Selecting both or either of the options "Thinning selection" and "Random selection"
• Using all the supported mask layer options
• Using a feature layer or a shape file as input training points
• Using a mask layer that all of the input training points don't fall within