Optimize showing cell numbers

Things that did to increase speed of program:
- Change finding cell centers function to a scipy implemented function
- Not calling show cell numbers for previous and next frame if it is not neccessary
- Not calling show cell numbers when clicking (in add region, add new cell, brush, and eraser)
- Remove unnecessary self.draw() call when calling show cell numbers
Note that updatind code dependencies (Python, Numpy, etc) also affect speed.