please make ROI to work

[alexlib]

@royekorolik
when we select ROI it means we define the region where to process the PIV - so the arrows should be calculated and presented only inside ROI

[alexlib]

Traceback (most recent call last):
File "/Users/alex/Documents/OpenPIV/openpiv_gui/package/piv_plot.py", line 262, in run
self.x += int(self.piv.xy_zoom[0][0])
TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'
/Users/alex/miniconda3/envs/openpiv_gui/lib/python3.6/site-packages/matplotlib/quiver.py:650: RuntimeWarning: Mean of empty slice.
amean = a.mean()