Train/predict on multispectral (4+ band) imagery

[PatBall1]

Introduce data readers that allow multispectral images to be used in training and prediction. There will be a possible issue with using pre-trained base models as has been done previously.

See: https://github.com/facebookresearch/detectron2/issues/698 https://github.com/facebookresearch/detectron2/issues/1292 https://github.com/facebookresearch/detectron2/issues/2062

Also: https://detectron2.readthedocs.io/en/latest/tutorials/data_loading.html