BUG: Replaced log(det(x)) with slogdet(x) to avoid numerical problems in loglike
ENH: Many typos fixed
ENH: Better PEP8 compliance in a few places
TYPO: Revert typo introduced in "Log likelihood"
ENH: Expanded test coverage for LS models

Added test coverage for some corner cases of WLS, which revealed a bug
in WLS.
Added test coverage for norm_resid, which was also incorrect.
Added exception checks for GLS sigma with wrong sizes.
Added test for QR where Q,R are user provided.  Revealed a bug in this code.
BUG: Fixed corner case bugs in WLS, GLS and norm_resid

Fixes bugs mostly in checks for correctness on input for GLS and WLS for
the sigma or weights.
norm_resid now produces the output described in the doc string
Remove stray print
Remove stray prints
BUG: Direct QR code not checking required inputs

The corner case of using pre-computed QR did not check all required
inputs were available.  Slight improvement in test coverage.
Added test for norm resid when model fits perfectly.
Fixes test for incorrect sigma array test.
Fixes test for direct vs computed QR eigenvalues.
TST: test_qr_alternatives, lower test precision
Merge branch 'linear_model_scale_1173' closes #1173