histogram: Option -r to set a reference range (1)

Read reference range from a file selected by option '-r'.
It must contain two lines with the minima for each column
on the first line and the maxima on the second line.
histogram: Option -r to set a reference range (2)

Set binning range such that the reference interval is
divided into the wished number of bins and then more
bins are added or removed to cover the data interval.
histogram: Option -r to set a reference range (3)

Allow for reference minimum smaller than data minimum.
histo2d: histo2d.c by Rainer Hegger added
histo2d: Whitespace corrections
histo2d: Integer cast stabilized

When dividing before multiplying, binning of integer data is unstable
due to errors within the double-precision of floating-point format.
histo2d: Output x,y values shifted to bin centers