Small fix to ndmapping_groupby_pandas function
Small fix to ndmapping_groupby_pandas function

Avoids hardcoding the 'Index' dimension used for NdElement types

vascotenner/holoviews,basnijholt/holoviews,ioam/holoviews,vascotenner/holoviews,ioam/holoviews,vascotenner/holoviews,basnijholt/holoviews,ioam/holoviews,basnijholt/holoviews