Changed settings for HoughCircles

param_1 = 2: Upper threshold for the internal Canny edge detector
param_2 = 32.0: Threshold for center detection.

Values from:
http://stackoverflow.com/questions/9860667/writing-robust-color-and-size-invariant-circle-detection-with-opencv-based-on
Test cases:
Overlay0001\1_image_A.tif: All cells detected
Overlay0002\1_image_A.tif: All cells but 2 detected
Overlay0003\1_image_A.tif: All cells detected
Overlay0004\1_image_A.tif: All cells detected
Overlay0005\1_image_A.tif: All cells detected but 1: overlap
Overlay0006\1_image_A.tif: All cells detected: possible overlap
Overlay0007\1_image_A.tif: All cells detected but 3: edges
Overlay0008\1_image_A.tif: All cells detected but 1