multiclosure internal dataset loader should use t0_covmat_from_systematics
Created by: comane
Bug in the current implementation. The internal_multiclosure_dataset_loader
function should take the t0 covmat from a single dataset rather than from data. In the previous implementation collect commands like the following were crushing because of shape incompatibility since t0 covmat was being constructed for the whole list of DataSetSpec's .
datasets_expected_bias_variance = collect( "expected_dataset_bias_variance", ("data",))