fix bug in spitter call (MAIN) (#357)

# CABLE

## Description

Following analysis in CABLE4 and discussion in #354 we should use total
SW down in the call to `spitter` and apply the resultant `Fbeam` equally
to the visible and near-infrared bands. The original intent (cable1.4)
of spitter is to evaluate the fraction of SW down that is diffuse in
nature - via the beam fraction `Fbeam`. This was evaluated once based on
the total shortwave down. From CABLE2 onwards spitter was called twice -
once for the visible and once for the near-infrared components. However
the function itself was not adjusted - the result is that beam fraction
is universally smaller than the original intent with unknown impacts on
the model trajectory.

This change set reverts the evaluation of `Fbeam` to operate on the
total SW down and then applies `Fbeam` equally to the two radiation
bands.

This has been implemented as a bug fix, not on a switch. This change may
require recalibration of the CABLE model (notably stomatal conductance
parameters, leaf nitrogen exponential and any other parameters that
impact GPP).

Fixes #355

## Type of change
- [x] Bug fix
- [ ] New or updated documentation

## Checklist

- [x] The new content is accessible and located in the appropriate
section.
- [ ] I have checked that links are valid and point to the intended
content.
- [ ] I have checked my code/text and corrected any misspellings


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📚 Documentation preview 📚:
https://cable--357.org.readthedocs.build/en/357/

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src/science/radiation/cbl_init_radiation.F90

@@ -378,8 +378,9 @@ SUBROUTINE BeamFraction( RadFbeam, mp, nrb, Cpi,Ccoszen_tols_huge, metDoy,     &
 
 
 ! Define beam fraction, fbeam:
-RadFbeam(:,1) = spitter(mp, cpi, metDoy, coszen, SW_down(:,1))
-RadfBeam(:,2) = spitter(mp, cpi, metDoy, coszen, SW_down(:,2))
+! #355 beam fraction defined using total SW_down; applies to VIS&NIR equally
+RadFbeam(:,1) = spitter(mp, cpi, metDoy, coszen, SW_down(:,1)+SW_down(:,2))
+RadFbeam(:,2) = RadFbeam(:,1)
 
 ! coszen is set during met data read in.
 WHERE (coszen < Ccoszen_tols_huge )