Handle boundary edge components and refactor iter_dem_instructions_include_correlations

[oscarhiggott]

Fixes a bug where boundary edges weren't always handled properly when they were a component of an error. i.e. handle_dem_error was called the same on edges and boundary edges:

} else if (num_component_detectors > 0) {
    // If the previous error in the decomposition had 1 or 2 detectors, we handle it
    handle_dem_error(p, {component->node1, component->node2}, component->observable_indices);

This resulted in an exception if the boundary edge was not already in the graph. Also refactors related logic slightly.

[smadhuk]

LGTM

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.14%. Comparing base (ae3f212) to head (7fef178).
⚠️ Report is 11 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
- Coverage   99.15%   99.14%   -0.01%     
==========================================
  Files           3        3              
  Lines         353      351       -2     
==========================================
- Hits          350      348       -2     
  Misses          3        3              

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