mule-convpp can't convert timeseries data.

Users on Monsoon3 have been cast adrift by the lack of a legacy copy of um-convpp. [see related discussion](https://github.com/MetOffice/simulation-systems/discussions/491)

This was previously used as a workaround to Mule reporting

> skipping field validation due to irregular lbcode

for fields containing timeseries data. What that error message doesn't tell users is that the field is subsequently treated as a variable resolution grid and then "mangled" to add variable resolution metadata such that it makes no sense to downstream processes which just don't read it. (At least I think that's whats happening...)

Mule needs to be updated to handle these gregorian timeseries files and produce readable output files.
This may also involve updating the validation section of Mule, which as indicated in the above message, it's currently skipping...