lot's of work on on_off.py
Rename on_off.py to poisson.py
add to test_on_off.py
Rename test_on_off.py to test_poisson.py
Add Sherpa fit statistics
Minor docs update
update poisson stats
Update Sherpa fit_statistics.py
Move fit_statistics one folder up
Add test_fit_statistics.py
update
Better variable names in background
add intro text to tevpy.stats docs
Rename statistics to stats

Because it's shorter and consistent with astropy
update
update
Rename statistics to stats

Shorter and consistent with astropy
update
update
Add unfinished test statistic image code