integration of agbd-lite dataset

[ghjuliasialelli]

Edits to integrate the AGBD-Lite dataset to the supported datasets.
Overview:

File(s)	Change	Notes
environment.yaml & requirements.txt	added h5py	necessary to load the dataset
pangaea/engine/evaluator.py	in RegEvaluator, evaluate() : implemented sparse backprop based on ignore_index	sparse backprop
pangaea/engine/trainer.py	in RegTrainer, compute_loss() and compute_logging_metrics() : implemented sparse backprop based on ignore_index	sparse backprop
New: configs/preprocessing/reg_resize.yaml		since images are 25x25
New: configs/dataset/agbdlite.yaml and configs/dataset/agbdlite-seg.yaml		one for regression, one for segmentation
New: pangaea/datasets/agbdlite.py and pangaea/datasets/agbdlite-seg.py		//

The edits to enable sparse backprop