Improve plot subcommand

Commit d554fb2c33321f971bc53ea464a1f48ce4042434 introduced the plot subcommand, but we should improve it a lot:

[x] automatic log-x / log-y detection: when x1 has a unit, use a log scale for y, implemented in commit b45d56cd1ddbaf78c8b6aed88e73d1d2c37175bf
[ ] improve x/y-log scale heuristic; the following grids shouldn't have an x/y log scale:
- [ ] ATLAS_DY_7TEV_LOMASS_EXT
- [ ] CMS_TTB_8TEV_2D_TRAP_TPT
[x] change the second panel to include, using errorbars, the PDF uncertainty of the main PDF set, added in commit 5947c513c074e0ebc20b4bdf75f2d6b8ba726647
[x] improve all legends of all panels, added in commit 5947c513c074e0ebc20b4bdf75f2d6b8ba726647
[ ] change the third panel to show the PDF uncertainties using stacked histograms
[x] do not show PDF comparisons if only one PDF set is given; implement in commit 846143003efae9d0cc05730536b17819b2280f25
[x] add a fourth panel showing the 'pull', implemented in commit 5947c513c074e0ebc20b4bdf75f2d6b8ba726647
[x] implement 2D/3D/... distributions by showing them as slices; added with commit bb493ef13059819b2f1db59ccde8e8dcdec718dc; ~~doesn't work for CMS_TTB_8TEV_2D grids (last slice is left out)~~ fixed in commit 30e047ef91a15a0c2e4a0ef30e3088a88cacb090
[x] if plotting multi-dimensional distributions, show the observables in which the plots are sliced; implemented in commit 46dc50bf65f9a49e4d34490cbdad7d14d783fdb6
[x] implement integrated cross sections (where there's just one bin); implemented in commit 0aa83ad9ae47aa45295ae78d985aa30568574d70

NNPDF / pineappl