update split transaction handling for csv export (#2973)

* Split transaction handling for csv export (#2973)

* refactor, zero-amt parents and add split_amount

change how child/parents are counted to remove dependence on `sort_order` being negative. for export, give parent transactions a 0 in the `amount` column, but add a new column with their `split_amount` for other programs to possibly import

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Co-authored-by: Zeus\Herb <herb@win.dows>
Co-authored-by: DJ Mountney <david.mountney@twkie.net>