Remove the max_steps_gap parameter and missing timestamp functionality in select_time_slice()

[dfulu]

In the slice_datasets_by_time() function, the call to select_time_slice() used for the satellite data uses max_steps_gap=2 - see here.

The value of max_steps_gap is entirely ignored in this function. This parameter is a carry over from ocf_datapipes and the slicing logic which uses this setting has not been ported across yet. Quite possibly we will never want to use this functionality since this was intended for use only in a production setting where data might be missing. We do this in the app now instead. During training, since we filter to t0 times where no satellite data may be missing, our current usage would never hit this anyway. We do not have the functionality in our t0 filtering to allow any missing time steps, so we should never need this.

I suggest we:

• Strip out this parameter and the placeholder _sel_fillinterp() function which it was intended to be used with.
• Plus remove the _sel_fillnan() function and the fill_selection parameter. I don't think we use these or will ever use these for training.