[vg6d_transform] Missing interval in cumulated output

[brancomat]

(maybe related to #98)

test data: tp_icon.tar.gz

cumulated output on total precipitation (or snow) apparently is missing the first interval, e.g.:

$ vg6d_transform --comp-stat-proc 1 --comp-frac-valid=0 --comp-step="0 12" --comp-full-steps tp_icon.grb qualcosa.grib
$ grib_ls qualcosa.grib 
qualcosa.grib
edition      centre       date         dataType     gridType     typeOfLevel  level        stepRange    shortName    packingType  
2            cnmc         20220612     fc           regular_ll   surface      0            12-24        tp           grid_simple 
2            cnmc         20220612     fc           regular_ll   surface      0            24-36        tp           grid_simple 
2            cnmc         20220612     fc           regular_ll   surface      0            36-48        tp           grid_simple 
2            cnmc         20220612     fc           regular_ll   surface      0            48-60        tp           grid_simple 
2            cnmc         20220612     fc           regular_ll   surface      0            60-72        tp           grid_simple 

I'd expect a 0-12 interval too (source data seems to be present) but it's missing.

[dcesari]

I introduced the n-th error in cumulation by differences when an interval is already ready. Fixed in b9d9f62.