[WIP] Contrast reserve

[WimDeMeester]

Adds the calculation of contrast reserve to PiFinder.
The value of the contrast reserve is shown in the object detail page:

Remarks:

• As the SQM is not yet calculated by the PiFinder, I take a fixed value of 20.15 for the SQM.
• The contrast reserve is just a number... For most users, this number does not mean anything. We still need to find out how to display the contrast reserve in a more meaningfull way

Contrast Reserve	Visibility
< -0.2	Not visible
-0.2 < CR < 0.1	Questionable
0.1 < CR < 0.35	Difficult
0.35 < CR < 0.5	Quite difficult to see
0.5 < CR < 1.0	Easy to see
1.0 < CR	Very easy to see

This pull request will solve #129