More engines for flow_view_deps

[moodymudskipper]

We chose {nomnoml} because it was the only one (with plantuml) to be able to render code, because of unescapable special characters with other tools.

However flow_view_deps() doesn't show code so probably many other tools might work too.

• We might have better cosmetics, like arrows pointing to the header not the middle of the block.
• We might have a better default order of items, which would help for issues like #124

[moodymudskipper]

OTOH bringing in plantuml brought in a lot of problems and inconsistencies, so this now looks to me like feature bloat. If we need something else for some reasons , the data element of the flow diagram output provides what's needed for other diagram tools.