Fix spill numbering scheme

[jdkio]

Fixes issue #164

Example where each entry is a time slice. After a few time slices it changes spill numbers

Reco_Tree->Scan("SpillNo")
*        7 *         0 *
*        8 *         0 *
*        9 *         1 *
*       10 *         1 *
*       11 *         1 *
*       12 *         1 *
*       13 *         1 *
*       14 *         1 *
*       15 *         1 *
*       16 *         2 *
*       17 *         2 *
*       18 *         2 *
*       19 *         2 *

[LiamOS]

Also confirming I saw the same issue in other branches. Thanks for fixing!