fix table chi2 grouped by

[scarlehoff]

When using the function groups_chi2_table, the group information is being dropped. The list of chi2 is indeed grouped by the group you are interested in but the dataset is saved as the group key and the total-per-group is not being used.

This is a fix to #1532 (which uses this function and which was the original bug)

Before:

        central_mean    npoints chi2_per_data   perreplica_mean perreplica_std  group
0       105.6      121     0.87      109.29      3.14      NMC_NC_NOTFIXED_DW_EM-F2
1       320.0      204     1.56      323.24      12.31     NMC_NC_NOTFIXED_P_EM-SIGMARED
2       30.12      33      0.91      31.253      4.01      SLAC_NC_NOTFIXED_P_DW_EM-F2
3       20.74      34      0.61      22.02       2.583     SLAC_NC_NOTFIXED_D_DW_EM-F2
4       465.1      333     1.39      474.0       19.4      BCDMS_NC_NOTFIXED_P_DW_EM-F2

Now:

        central_mean    npoints chi2_per_data   perreplica_mean perreplica_std  dataset group
0       105.679      121     0.87      109.26      3.14     NMC_NC_NOTFIXED_DW_EM-F2        DIS NC
1       320.00       204     1.56      323.24      12.3     NMC_NC_NOTFIXED_P_EM-SIGMARED   DIS NC
2       30.122       33      0.91      31.25      4.012    SLAC_NC_NOTFIXED_P_DW_EM-F2     DIS NC
3       20.740       34      0.61      22.04       2.58     SLAC_NC_NOTFIXED_D_DW_EM-F2     DIS NC
4       465.127      333     1.39      474.08       19.      BCDMS_NC_NOTFIXED_P_DW_EM-F2    DIS NC
5       945.59       725     1.30      963.74       23.                                      DIS NC