Add SKeMa models

[tayden]

This PR aims to add both SKeMa models into kelp-o-matic.

To Do:

• Add model that uses S2 data only
• Add model the uses S2 data plus depth data
  • Refactor code to add additional CLI arg for path to depth data?
• Use weights from a public HTTP source for automatic downloading (HuggingFace, likely)
• Refactor code to allow different data inputs via some abstract Reader class (e.g. .SAFE files)
• Add attribution for Spectral Lab and/or Mohsen Ghanbari

Currently, the model weights are private.