histond: Copy histo2d for n-dimensional histograms

The revision e8285b86 without option -r is chosen.
histond: Allow selection of several columns
histond: Calculate histogram (third column) only
histond: Allow for arbitrary number of columns (1)
histond: Allow for arbitrary number of columns (2)
histo2d: Option -L for relative frequencies
histond: Option -L for relative frequencies