Related to issue #53 ROI selection

[LuisOlivaresJ]

Here are the additions:

1. Adding a new callback function (key_c_pressed) to catch a ROI when the key c is pressed.
2. The ROI size (in mm) is shown in the terminal when RectangleSelector changes.
3. New style for RectangleSelector.

The code I used for tesitng (in Anaconda Powershell) was:

from omg_dosimetry.calibration import LUT
from pathlib import Path
import matplotlib.pyplot as plt
from omg_dosimetry import calibration

my_path = calibration.from_demo_image()
doses = [0.0, 100.0, 200.0, 400.0, 650.0, 950.0]

lut = LUT(my_path, doses, crop_top_bottom = 650, film_detect=False) 

[jfcabana]

That looks great Luis. Very nice addition!

[jfcabana]

I'll let you go ahead and merge it.