pydantic model of sentinel2

[d-v-b]

With a pydantic model of sentinel2 data, we can do 2 things:

• define the geozarr conversion as a mapping from 1 pydantic model to another
• catch changes to the structure of sentinel2 data, which would otherwise silently propagate and potentially pollute the conversion process.

[d-v-b]

closed via #64