Adding the "special bin" option to the measurement

[vukasinmilosevic]

Certain bins for the double differential measurement have a special first bin definition. One example of this is the pt_j1 vs pt_j2 distribution, where the first bin represents the scenario where we have Njets < 2.

Before implementing this, we need to discuss the definition of the "outside the fiducial space" cut for this bin (wether it is simply Gen_nJets >= 2 or something else).

[vukasinmilosevic]

To be added as well: Support for for single 2D bin, i.e. |20|30| vs |30|35| - we don't need it for the measurement, as there are no such bins, but it would be good to add it for code reusability.