BARPA-R screenlevel specific humidity instead of lowest ml

At this stage of the post-processing of BARPA-R screen level specific humidity (i.e., qsair_scrn) instead of lowest model level specific humidity is used. qsair_scrn is known to have issues in some time steps and it might be good to use lowest model level data instead.

However, lowest model level data is unavailable on disk (only on tape) and would need to be retrieved first. The post-processing mapping would also need to be updated.

Start by identifying the BARPA-R post-processing stage and mapping that currently uses qsair_scrn, then determine how to retrieve the lowest model level data from tape. Update the post-processing mapping to use that data, and confirm the resulting screen-level specific humidity no longer depends on qsair_scrn.

Written by the indexing model from the issue text.