BUG: fix weight handling, fails in some cases, see #987
TST: predstd move __main__ to a test module
BUG: wls_prediction_std: fix if weight is scalar
CLN: pep-8
TST: wls_prediction_std: additional tests
TST wls_prediction_std add tests for OLS and confint
Merge pull request #1582 from josef-pkt/fix_prediction_std

BUG: wls_prediction_std fix weight handling, see 987