New plugin to calculate the standard geopotential height

[mo-robertneal]

A new plugin is added to generate the standard geopotential height using a lookup table, where values are read based on an input pressure level.

Usage:

import iris
import improver
incube = iris.load_cube(
    "/data/users/eppdev/EPP_data_cache/global/20260319T1200Z/20260325T1200Z-PT0144H00M-geopotential_height_on_pressure_levels.nc")

from improver.standard_geopotential_height.standard_geopotential_height import (
    StandardGeopotentialHeight,
)
plugin = StandardGeopotentialHeight()

outcube = plugin(incube)