Add example: computing logical error rates of individual logical operators in a memory experiment.

[perlinm]

From discussions with Jonathan Baker

[perlinm]

Looks like this should be doable by passing appropriate options to sinter, probably as an option in collect. From https://pypi.org/project/sinter/#how_it_works:

custom_counts (json[Dict[str, int]]): An optional field that can store a dictionary from string keys to integer counts represented in Java Script Object Notation. The counts can be a huge variety of things, ranging from per-observable error counts to detection event counts. In general, any value that should be added when merging rows could be in these counters.

[perlinm]

See count_observable_error_combos at https://github.com/quantumlib/Stim/blob/main/doc%2Fsinter_api.md

[HaoTy]

FYI, how tqec does it: https://github.com/tqec/tqec/blob/main/src/tqec/simulation/split.py.