ENH: add feature to exclude tree crowns from agricultural parcels

[theroggy]

Some agricultural parcels, especially narrow grassland parcels, can have a relatively large percentage of their area attributed to trees crowns that hang over the parcel.

It would be useful to test if it is beneficial to exclude these overhanging tree crowns from the parcels to improve classification accuracy.

The feature could be something like this:

• add the option to specify a path to a "erase layer" file to the config (to be specified in the run_task config file)
• in a prepare function, before the negative buffer is applied:
  • if a "erase layer" file exists, the agricultural parcels input file needs a column "ignore_erase_layer"
  • create tree_crowns_erase_forest_prc file by erasing all agricultural parcels with "ignore_erase_layer" True from the erase_layer" file
  • erase the tree_crowns_erase_forest_prc from the input agricultural parcels