Implement `np_batch_processor.EncodeSpaceTime`

[JackKelly]

Use the code I wrote in December as a start. Planning to use relative position encoding (because the most important function of absolute position is the Sun's position; and we'll give the Sun's position explicitly in the query).

patch the imagery first. See issue #32.

Implementation ideas:

• [x] implement as an np_batch_processor, which adds encodings for all modalities
• [x] Need to add np_batch_processor after to_numpy in NowcastingDataset.
• [x] Each modality will create BatchKeys like x_osgb, y_osgb, time_utc, and these will be masked with NaNs where appropriate (need to do this for PV).

PATCH_SIZE = 4
dataset = NowcastingDataset(
    data_loaders=[
        PV(),
        HRVSatellite(
            transforms=PatchSatellite(x=PATCH_SIZE, y=PATCH_SIZE)  # uses DataArrayCoarsen.construct
        ),
    ],
    xr_batch_processors=[
        SelectPVSystemsNearCenterOfImage(),
    ],
    np_batch_processors=[
        EncodeSpaceTime(),
    ],
)