Handling AIFS Ensemble Data in CSET

This issue captures development for handling AIFS ensemble data in CSET.
AIFS data is provided on a public html via ECMWF at https://data.ecmwf.int/forecasts/, in GRIB2 format.
The files appear to be structured as follows;

`20260414000000-72h-enfo-cf.grib2, i.e. %Y%m%d%H%M%s-%Nh-enfo-cf.grib2` containing control member data
`20260414000000-72h-enfo-pf.grib2, i.e. %Y%m%d%H%M%s-%Nh-enfo-pf.grib2` containing ensemble member data

Data in pf looks like the following in Xconv:

Where cf is similar, except just contains the control member (member numbered 0).

On the command line, iris cannot load the cube directly without a callback because of some mis-aligned metadata with error `iris.exceptions.TranslationError: Product definition section 4 has different types of first and second fixed surface`.

I am going to develop some standalone code that uses a callback to handle the data/filter - my suspicion is that soil variables are causing the problem with levels, which we won't use anyway. After this, I'll look at what is suitable for a generic callback in CSET.