Draft: Add 2poi option

Option to have 2 poi fit(2D scan) added to likelihood_scan.

• Options added: --do_2Dscan, --additional_poi_min, --additional_poi_max, --additional_poi_step [need to specify all 4 if you want to run 2d scan] Names are temporary, will come up with something shorter. --poi "poi_one,poi_two"

To do:

• likelihood.py could maybe be simplified how the points to scan are passed
• Could generalise to 3D or any number of POI, the limitation now is just defining the points of the additional poi to scan. Could be solved by --min/--max/--step getting a list of values similar as --poi.
• Run more tests (Already did some simple test for the 2D and 1D case and the results seem reasonable to first order.)
• extended_model.py: poi has been changed to list now, need to updated in all relevant places!