Merge 2d histogram plotting codes

[jllanfranchi]

Plotting itself--at least for maps, if not other things--should be handled by utils/plotter.py. If that doesn't do something it should, then that functionality should be added to plotter.py and not duplicated in multiple modules.

Places where plotting seems duplicated:

• utils/tests.py
• analysis/profile_scan_postprocess.py
• analysis/nmo_blindfits_postprocess.py
• analysis/hypo_testing_postprocess.py (these plots seem somewhat orthogonal to histogram plots, but should double check that's the case)
• core/map.py (we need a method that will simply plot a map or map set attached to these objects; but can be replaced by proper call(s) out to plotter.py module)
• scripts/make_systematic_variation_plots.py (also possibly orthogonal to 2d hist plotting)
• scripts/make_asymmetry_plots.py (might be unnecessary, as scripts/compare.py should handle this. Maybe need to add some command-line options to enable/disable plotting of non-asymmetry plots, etc. to the latter, though)

[thehrh]

While many of the above modules/scripts are gone, there is still plotting duplication in utils/tests.py.
Let's consider removing that module altogether, since it contains "Functions to help compare and plot differences between PISA 2 and PISA 3 maps".