In this mode, the user can specify a QGIS expression for the
lower and upper value corresponding to raster bands, using
variables like @band, @band_name and @band_description.
E.g
@band * 100
Can be used when each band represents a 100 m vertical slice
of data.
The expression will be evaluated when required to determine
the actual elevation range corresponding to each band.
This differs from the existing "Fixed Elevation Range Per Band"
mode in that "Fixed Elevation Range Per Band" requires users
to manually enter an elevation for each band separately,
and these values are then treated as constants. That mode works
best for rasters with non-regular steps in the band
elevation values, while this new mode is better for regular
band elevation steps
