Add ISPRS Potsdam segmentation dataset

[pierreadorni]

Added dataset class and config for the ISPRS Potsdam dataset, following the typical settings used by the community:

• train and val subsets follow the original ISPRS contest
• images tiled from 6000x6000 to 512x512 tiles with overlap of 128
• Clutter category is ignored during the loss computation
• We use the IRRG version, so bands B8-B4-B3

The images are converted from TIF to PNG during the download step to lower disk usage.

example image from the train subset: