Revert "Merge branch 'tdavies__weight_cmbchisq' into 'spiir-O4-EW-development'"

This reverts merge request !200 (merged)

I forgot to push a couple of fixes that were included in the test results. I'll quickly revert, push the full changes, then make that a new MR.

The merged version I believe compiles, but breaks if run (from memory with/without the feature flag).