REL-1501: Storing Scale Interpolation in TPE and code cleanup.

Unfortunately, the thing is that the only information I have at this point is the scale as represented by a `float` (as obtained from the `DivaGraphicsImageDisplay.getScale` method) and I need to look up the radio button based on that to set it to selected. The `DivaGraphicsImageDisplay.setScale(float)` method is called in no less than 12 places, I believe, so trying to change this would be quite a hassle, possibly with far-reaching consequences. The other thing I could do is just iterate over the keys in the HashMap myself and compare with the scale with a small margin of error (`1e-x` for some `x` sufficiently small to avoid any false positives) to try to avoid any false negatives.

Any other ideas / comments?