Refactor graph-related modules + introduce `fastflow`

[EarlMilktea]

my suggestions

• completely remove linalg stuff
  • drop sympy / galois from requirements.txt
• refactor OpenGraph to be able to have PauliMeasurement inside
• implement mutual conversion between Pattern and OpenGraph
• simplify/isolate modules
  • remove generator and make them methods of graph or flow
  • remove extraction and make them methods of graph
  • remove Pattern API from visualization: draw from graph directly
  • remove from_pattern / from_graph stuff if appropriate
  • define a dedicated module for complex pattern manipulations
    • complex = go back and forth between pattern and graph

[EarlMilktea]

@thierry-martinez @shinich1

I'll complete this issue immediately after merging TeamGraphix/swiflow#18 : this is necessary to correctly reimplement xxxflow_from_pattern functions.