Move to (faster) graph based Span parsing

[matiasdahl]

Run times mnist demo pipeline "run-all-tests-and-pipeline"-job

Significant speedup for "Render Gantt and dependency graphs" (16mins -> 42 sec, !!!)

Before

After

Run times for Github action "Run unit tests" step in CI-pipeline for pynb-dag-runner repo

Large variety and no clear speedup.

Last 5 runs in development branch

• 5:14 #675
• 4:24 #671
• 4:28 #669
• 4:26 #667
• 4:51 #665

5 runs after commit 29900d5 in this PR

• 5:06 #689
• 5:32 #690
• 4:36 #691
• 5:02 #692
• 4:26 #693