make inkscape command configurable

and fix b64 treatment of PDFs
simplify ConvertFiguresTransformer

it will always run *before* extract figures
add pdf to latex display priority
add data_pdf block to latex templates
Merge pull request #3695 from minrk/nbconvert

fix SVG2PDF

there were some logical issues, with SVG2PDF assuming it was run
both before and after figure extraction.

Also makes the inkscape command configurable,
in case it doesn't work some places.

Adds pdf block for output in latex templates.

closes #3693