[ncl-talk] Add new options for dim_cumsum

[Dave Allured - NOAA Affiliate]

NCL team,

I would like to suggest several new options for the dim_cumsum family of
four functions.  The current three options are:

opt = 0 ; when _FillValue is encountered no additional summing will be
performed.
opt = 1 ; when _FillValue is encountered set element to _FillValue and
resume summing.
opt = 2 ; treat _FillValue as numeric zero when summing.

Frequently only the final array slice of the customized summing operation
is wanted.  So I propose several new options:

opt = 4 ; when _FillValue is encountered, return _FillValue.
opt = 5 ; return sum only up to the first encountered _FillValue.
opt = 6 ; return sum only following the last encountered _FillValue.
opt = 7 ; treat _FillValue as numeric zero, and return only the final sum.

The first three existing options return an array of the same type and
dimensionality as *x*.  These four new options would return an array of one
lesser rank than x; the summing dimension would be removed.

Options 4, 6, and 7 would return exactly the same result as taking the
"last" array slice on the summing dimension, of options 0, 1, and 2
respectively.

Option 5 would be special.  Summing would end on the FIRST encountered
_FillValue, and that result would be returned.  Option 5 would be exactly
the same as option 6 with reversed input ordering on the summing
dimension.  Option 5 is included mainly for symmetry and completeness.

To be consistent with current option 1, _FillValue is explicitly returned
when:

opt = 5 and the first input element is _FillValue.
opt = 6 and the last input element is _FillValue.

These new options would simplify user code in some cases, such as Rashed's
case today.  In large array applications, these options could also reduce
memory consumption and run time.

--Dave
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