Add WorldStrat dataset

[nilsleh]

This PR adds the WorldStrat dataset for Super Resolution.

Dataset features:

• High resolution (1.5m/pixel) Airbus SPOT 6/7 imagery with RGBN channels
• Low resolution (8x lower) Sentinel 2 L1C and L2A
• globally distributed areas of interest around the world

Dataset format:

• pixel dimensions vary across AOI tiles
• all modalities are 'tif' files except for 'hr_rgbn' which is 'png'
• 'hr_ps', 'hr_pan', 'hr_rgbn' are high resolution data of pixel dimension
• 'lr_rgbn' is low resolution data of pixel dimension and roughly 4x lower resolution than 'hr_rgbn'
• 'l1c' and 'l2a' are Sentinel-2 data with 13 and 12 bands respectively and roughly 8x lower resolution than 'hr_rgbn'

I think the plotting function can be improved.

Waiting for a fix: worldstrat/worldstrat#9

@ivanorsolic @jucor Thank you for open-sourcing the dataset. This PR aims to integrate and facilitate easy data loading with torchgeo. We would welcome any comments etc about other useful/required processing steps or mistakes you might find here.