Tidy up NWP time selection

[dfulu]

Both of these functions do essentially the same thing:

• https://github.com/openclimatefix/ocf_datapipes/blob/main/ocf_datapipes/transform/xarray/convert_to_nwp_target_times.py
• https://github.com/openclimatefix/ocf_datapipes/blob/main/ocf_datapipes/transform/xarray/nwp_dropout.py

It would be cleaner just to have the dropout one with a default dropout value of 0.

Also it would be more natural to have this at ocf_datapipes/select