BUG: viz: black background in PDF and SVG output
Remove trailing print
DOC: correct docstring
ENH: make it easier to use axes in slicers
COSMIT: nicer vizualization example
MISC: remove transparency

Using transparency is a bad idea because if you save as figures as PDFs,
the corresponding PDFs will rely on transparency in PDF which
cannot be translated to EPS (EPS does not have the notion of
transparency). As a result, publishers like Elsevier will rasterize the
figure, often in a very crude way. People keep getting bitten by this
effect. It is better to change the defaults.
Merge remote-tracking branch 'gael/viz_black_bg' into main-master

A set of small fixes to plotting:

* black-bg now works with pdf/svg outputs
* removed a debug print
* docstring fixes in plot_anat/plot_map
* minor tweaks to make it easier to specify axes
* remove default transparency for compatibility with EPS

* gael/viz_black_bg:
  MISC: remove transparency
  COSMIT: nicer vizualization example
  ENH: make it easier to use axes in slicers
  DOC: correct docstring
  Remove trailing print
  BUG: viz: black background in PDF and SVG output

Conflicts:
	examples/labs/need_data/viz.py
DOC: fix typos in visualize