Make sklearn dependency required

[ashnair1]

Not sure if sklearn is now a mandatory requirement or an optional one for datasets. If the latter, a note needs to be added to cyclone.py and sen12ms.py

Fixes #230

[adamjstewart]

Thanks! I think my plan is to make everything that requires a top-level import to be a required dependency, meaning that only packages that are lazily imported can be optional. This will allow all files in TorchGeo to be imported as long as the core dependencies are installed. I'll make sure this happens before the next release.