Remove method Pattern.graph

[matulni]

This commit removes the method Pattern.graph which is not correct as it does not standardize the pattern. Where appropriate it was replaced by Pattern.to_opengraph().graph, including unmerged branches in reverse dependencies.

Method Pattern.isolated_nodes is rewritten to avoid constructing the open graph as it's not necessary.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.63%. Comparing base (a075ac7) to head (baeabb2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #571   +/-   ##
=======================================
  Coverage   90.62%   90.63%           
=======================================
  Files          49       49           
  Lines        7481     7475    -6     
=======================================
- Hits         6780     6775    -5     
+ Misses        701      700    -1     

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