lfric2um may need to regrid time series

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

To support generation of LBC forcing conditions, lfric2um will need to take and regrid output from several model timesteps.

Depending on how the output from the LFRic model is implemented, this may be a single time-series or it may be separate times in different files. The former will require support for reading time-series.

Briefly, whether fields are in separate files depends on R2O. The benefit of output from individual files is that it allows regridding to take place while the original model is still running. But that depends on the ability to safely process the field while the model is still running (knowing when XIOS has finished writing a file is an unsolved problem).

Therefore, there is a reasonable likelihood of needing to regrid time-series.

### Further details of the issue.

_No response_

The issue names lfric2um, R2O, and XIOS but no files or tests. Start by tracing how lfric2um currently reads and regrids model output, then establish whether multi-timestep input is a single time-series or separate files; done requires a decided implementation path and clear handling of file-writing safety.

Written by the indexing model from the issue text.