Update histogram.py
Update histogram.py

Update variable names upper/lower_range to upper/lower_bound
Make the default values of upper/lower_bound to be min/max of the original data.

stiphyMT/plantcv,stiphyMT/plantcv,danforthcenter/plantcv,stiphyMT/plantcv,danforthcenter/plantcv,danforthcenter/plantcv