fix: per-channel yield uncertainty contribution from staterror-staterror terms

[alexander-held]

The model_utils.yield_stdev implementation contained an optimization for staterror-staterror cross terms, which have no contribution when calculating per-bin uncertainties:

cabinetry/src/cabinetry/model_utils.py

Lines 303 to 307 in f34c73a

	if (
	labels[i_par][0:10] == "staterror_"
	and labels[j_par][0:10] == "staterror_"
	):
	continue # two different staterrors are orthogonal, no contribution

When the functionality to calculate per-channel uncertainties was added in #189, the optimization should have been updated, since these cross terms no longer cancel out for the per-channel calculation. This now fixes the bug in per-channel yield uncertainties by removing this performance optimization completely.

While this does make the calculation slower overall, that is not a concern since it will be replaced completely by #316 afterwards, resulting in a significant speed-up. This intermediate fix is just meant to split the development up into steps that are hopefully easier to follow when looking back at this in the future.

resolves #323

* fix calculation of per-channel yield uncertainties
* calculation was previously missing contributions from staterror-staterror terms

[codecov]

Codecov Report

Merging #324 (d41065f) into master (f34c73a) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #324   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         1889      1886    -3     
  Branches       306       305    -1     
=========================================
- Hits          1889      1886    -3     

Impacted Files	Coverage Δ
src/cabinetry/model_utils.py	100.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f34c73a...d41065f. Read the comment docs.