Plot updates

This PR is meant to include a collection of updates of plots after receiving feedback from the 4b boosted approval. In particular, the changes aim to improve visual guidance for reading results, compliance with official figure guidelines, overall clearness, and to issue warnings in case uncommon conditions are detected (Nan's in fits, negative dnll2 values, etc.).

• Likelihood plot: warn about negative dnll2 values that appear when combine used a local minimum to compute likelihood shifts
• Likelihood plot: add option to recompute the POI minimum
• Likelihood plot: use better default for minima when doing log plots
• Likelihood plot 2D: show white pixels by default for grid points with failed fits, print a warning
• Likelihood plot 2D: add option to fill in values from neighboring pixels (only recommended when happening very rarely and at "unimportant" locations)
• Likelihood plot 2D: show best fit value only on request
• Likelihood plots: warn about cases where best fits from combine and scipy diverge
• Likelihood plots: add option to vertically shift dnll2 values when negative ones were detected
• Exclusion plot 2D: show best fit value only on request (just flip the default)
• Exclusion plot 2D: add "brazil" mode
• All: Remove "Preliminary" label when --paper flag enabled
• All: Make "CMS" logo larger and move into plot when possible
• All: Use "68/95% expected" instead of "±1σ" where applicable
• All: maybe make legend position configurable via a single parameter When doing this generically, this could lead to severe layout conflicts in most plots and needs different additional changes per plot.
• S-over-B plot: change pre/postfit labels depending on --unblinded flag.
• Update fake plots in docs showing new style Coming in additional PR (@mfackeld)

@slaurila