histogram: Internalize rescale_data() in main code

The check for an empty interval is already covered
by the check for zero variance in variance().
histogram: Uniform binning of integer data assured

Try 'python -c "for i in range(51): print i" | ./histogram' to see
that the determination of the corresponding bin number is more precise
and system independent after reordering of division and multiplication.
histogram: Variable 'max' renamed to 'interval'

Now the maximum is min+interval instead of min+max.