Minor update to Met routines

[Whyborn]

@juergenknauer (and @mcuntz?) A minor recommendation, around some updates to the Met routines.

The recent work around the BIOS code introduced some changes in how the new Met routines operate. We're moving to a philosophy of "if you supply it (as an input), we will try to use it". This affects the way we handle the diffuse radiation fraction- if a valid string for a meteorology (which means anything that is not "None"), we will try to read it. It shouldn't have any effect on running TRENDY, but I'd recommend a minor update to run_cable.sh around here.

When you move to the current tip of CABLE-POP_TRENDY, I'd recommend setting

if [[ ${read_fdiff} -eq 1 ]] ; then
    fdiff_bool=.true.
    fDiffFile = "${MetPath}/fd/fd_<startdate>_<enddate>.nc"
else
    fdiff_bool=.false.
    fdiffFile = "None"
fi

and

cat > ${tmp}/sedtmp.${pid} << EOF
    ...
    fDiffFile = "${fDiffFile}"
    ...
EOF

in run_cable.sh.