Use irregular spectral grids, if available,
for calculations, interpolating spectra to regular grid before output.
If generating look-up tables (TAB Flag)
the output is also on the irregular grid.
From RFM v4.33
onwards, an irregular grid can also
be constructed from the set of spectral points in the look-up tables
supplied in via the *LUT section of the
Driver Table so an explicit irreg.grid input file is not required.