Speed up `Manager.sample_spatial_and_temporal_locations_for_examples()` by splitting `shuffled_t0_datetimes` across multliple processes

[JackKelly]

but wait until we've implemented #305, as #305 is likely to be sufficient!

[peterdudfield]

I couldnt see an obvious way for #305, so i did #304 first

[JackKelly]

I'm just re-opening this issue because we might still need this (but not sure yet!)