Allow users to specify geographic bounds

[JackKelly]

Detailed Description

Implement config fields to allows users to specify the geographic bounds of the dataset (e.g. "just produce examples over the UK.")

Context

Soon, we should have satellite intermediate data that spans the whole SEVIRI geographic extent. But, for now, we just want to focus on the UK. But it'd be great if changing the geographic bounds was a simple config change, rather than having to change the code.

Possible Implementation

The config yaml could include something like:

geographic_bounds:
    x_max = float
    x_min = float
    y_max = float
    y_min = float

These geographic bounds could be used by downstream code, such as nowcasting_dataloader.utils.position_encoding.get_seviri_rss_bounds().