`NWPDataSource._get_time_slice` should use NWP init time closest to `t0` for forecast timesteps

[JackKelly]

Detailed Description

At the moment, we use NWPs with an init time at or just before the start datetime of the historical data for each example.

Instead, we should use the NWP init time most recent to t0. This should slightly improve the NWPs for the forecast time period. Although this is most relevant if/when we use NWPs for more than a few hours.

Possible Implementation

This logic was implemented in an earlier version of NWPDataSource._get_time_slice but I removed it because it was a little slower (and, back then, I was still trying to load data on-the-fly during training). But that slightly increase in runtime shouldn't be an issue now that we're pre-preparing batches.