Support torchgeo.datasets.NonGeoDataset

[Pale-Blue-Dot-97]

Is your feature request related to a problem? Please describe.
Not all datasets in torchgeo are GeoDatasets -- some are NonGeoDatasets that do not use geospatial information. I am having issues with CRSs when using a global, pre-patched dataset like SSL4EO-S12. torchgeo already supports this dataset with a torchgeo.datasets.SSL4EO-S12 but is a NonGeoDataset. However, a lot of minerva is designed to work with GeoDataset, particularly the paired sampling.

Describe the solution you'd like
Generalise minerva to work with NonGeoDataset. In particular, there will need to be versions of PairedDataset and PairedUnionDataset that work with NonGeoDataset and the associated paired sampling.

Describe alternatives you've considered
The issue encountered with the CRS in SSL4EO-S12 relates to the projection system resulting in patches sampled with missing data. This could alternatively be avoided by rejecting any samples containing zeros -- but this would be an inefficient and messy implementation.