Fixed strict type error
Fixed strict type error

The test test_plantcv_background_subtraction_different_sizes had a
division that could return float values where integers are needed.
Python3 is strict about this, so the values are now cast as integers.

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