check for nans in nwp batch data

[peterdudfield]

Detailed Description

Check for nans in the nwp batch data

Context

• good to have clear errors, that fail hard

Possible Implementation

• perhaps put i here - https://github.com/openclimatefix/ocf_datapipes/blob/main/ocf_datapipes/batch/merge_numpy_examples_to_batch.py#L93
• would be good to have seperate function to check, which can be tested

[glitch401]

this seems similar to isses I addressed, can I take this up?