HCx_se are manually cached, cov_HCx are only available after calling se
use cached attributes for both instead
refactored in PR #1189
REF: RegressionResults convert HCx to cache_readonly, closes #1206
docstring of RegressionResults hasn't been changed yet.
I didn't convert het_scale into a cached property, I don't think they are of much interest without the corresponding covariance.
@jseabold Is there a special reason you didn't use cached properties in the first place, or does eeeb326 look ok with you?
DOC: adjust docstring for cov_HC# and HC#_se for previous commit, see #…