Pauli-flow extraction from pattern #432
Description
The class Pattern have methods extract_causal_flow and extract_gflow, but extract_pauli_flow is missing. This causes, in particular, a regression in #181 where the visualization cannot display a flow on graphs with Pauli measurements. Before this PR, if graphs with Pauli measurements happened to have a gflow when their Pauli measurements were interpreted as planar, the visualization was able to show their gflow while displaying the Pauli nodes, but we no longer do that since we have eliminated implicit conversions.
Extracting Pauli flows does not seem trivial, since the anachronical corrections do not appear in the pattern.