Sentinel 2 input spec and retrieval

[lillythomas]

General specifications for v0

Initially, we've landed on selecting the 10-20 meter bands from Sentinel 2, plus the SLC band for tile-level cloud cover thresholding.

The bands to be included:

• B2 (490 nm), 10 meter
• B3 (560 nm), 10 meter
• B4 (665 nm), 10 meter
• B8 (842 nm), 10 meter
• B5 (705 nm), 20 meter
• B6 (740 nm), 20 meter
• B7 (783 nm), 20 meter
• B8a (865 nm), 20 meter
• B11 (1610 nm), 20 meter
• B12 (2190 nm), 20 meter
• SLC (not for training)

We will use the L2A data product right now, but note that we discussed internally it may be worthwhile to explore a more applicable atmospheric correction method for water-related downstream tasks in future versions of the model.

Retrieval

We will build on what @yellowcap started in #11, adjusting per the aforementioned specs.

[weiji14]

Closing as implemented in #27, feel free to comment further or reopen if we want to modify the specification further.