Add SenBench-Biomass-S3 dataset from SentinelBench

[wangyi111]

This PR adds SenBench-Biomass-S3 dataset from SentinelBench.

The SenBench-Biomass-S3 dataset is a level-3 dataset from the SentinelBench benchmark.

It contains Sentinel-3 OLCI images and CCI biomass maps for the biomass regression task.
It supports both static (1 image / location) and time series (1-4 images / location) mode, the former is used in the original benchmark.

Dataset features:
* task: dense regression
* # samples: 3000/1000/1000 (train/val/test, static mode)
* image resolution: 96x96 (GSD 300m)
* label resolution: 282x282 (GSD 100m)

Dataset format:
* images: 21-band Sentinel-3 OLCI images (GeoTIFF)
* labels: biomass maps (GeoTIFF)

I made this PR separate from the previous ones..