data subsetting and spots coordinates rounding

[LouisK92]

• Find out: does the spot x-coordinate "0" refer to the start of pixel with x idx "0" or to it's center?
• Adjust subsetting functions accordingly: e.g. for tiling df["x"].round() >= x_min & df["x"].round() < x_max (x_min, and x_max are integers --> first pixel=x_min, last pixel=x_max-1)