Zero out syndrome in the support of UserGraph boundary nodes

[oscarhiggott]

Fixes an issue where adding a detection event on a node marked as a boundary node in the Python API led to:

ValueError: No perfect matching could be found. This likely means that the syndrome has odd parity in the support of a connected component without a boundary.

E.g. This breaks tests in qiskit-qec: qiskit-community/qiskit-qec#276

The fix is just to zero out the syndrome in the support of these boundary nodes, since the algorithm already handles the boundary correctly without these additional detection events.

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (feed3ce) compared to base (df64928).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #40   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines          299       299           
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  Hits           299       299           

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[oscarhiggott]

Fixes issue #41