Add geopotential height at 700 hPa diagnostic (processed__geopot)

[Zubair Maalick (zmaalick)]

Implements issue #604: geopotential height at 700 hPa (zg700), output 6-hourly. Reuses the already-computed multi-level plev__geopot by slicing its 700 hPa level (index 5 of the pressure_levels axis) via an XIOS axis zoom, mirroring the existing processed__pstar two-step extraction pattern.

• grid_def_main.xml: new pressure_levels_700_face_zoom grid.
• field_def_diags.xml: zoomed__geopot (level slice) and processed__geopot (face field, name=zg700, standard_name=geopotential_height, unit=m).
• file_def_diags_oper_nwp_gl.xml: new 6-hourly file group requesting it.

Registry-only change; no Fortran/kernel/algorithm edits.

PR Summary

Sci/Tech Reviewer:
Code Reviewer: Benjamin Went (@MetBenjaminWent)

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[github-actions]

Hello Zubair Maalick (@zmaalick)! 👋

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[iboutle]

Hi Zubair,

I'm a bit puzzled for what the motivation for this diagnostic is - are you able to give any further details about why we need a specific diagnostic of the 700hPa geopotential that is separate from the general geopotential height on pressure levels diagnostic already available?

In particular, you're adding this diagnostic to the operational suite diagnostics list, but I can't see anything equivalent in the operational UM diagnostics list?

As an aside, I don't think this is the correct way to produce this diagnostic if indeed we do need it (we can't assume that 700hPa will always be level 5, because the pressure level axis is user defined), but it will be easier to establish how to approach it when I understand the requirement better.

Thanks

Ian