DOC: complete stats.boxcox docstring.  Add add boxcox functions to refguide.

Also do a PEP8 cleanup of `boxcox`.
TST: add tests fox stats.boxcox.
DOC: complete stats.boxcox_llf docstring.  Also PEP8ify that function.
TST: add tests for stats.boxcox_llf.  Also add some input parameter handling.

Add also a reference to the original paper of Box and Cox.
DOC: clean up and document stats.boxcox_normplot.

Also some PEP8 cleanups in related functions.
TST: add tests for stats.boxcox_normplot.
BUG: fix bug in confidence interval estimation of stats.boxcox.
TST: finish stats.boxcox_llf test for 2-D input.  Fix bug in it.

boxcox() doesn't accept 2-D input, leave it like that.
ENH: add tests and docstring for stats.boxcox_normmax().

Also refactor boxcox and boxcox_normmax so MLE can be used from the latter.

Changes the `brack` keyword in boxcox_normmax; it differed from that used in
boxcox(), keeping boxcox() results unchanged is preferred here.

This finishes the Statistics Review for all Box-Cox functions:
Closes gh-680
Closes gh-681
Closes gh-682
Closes gh-683
Closes gh-684
MAINT: address review comments from Warren on pr-3080.
MAINT: fix grammar in stats.boxcox docstring.

Addresses review comment on gh-3080.

[ci skip]
Merge pull request #3080 from rgommers/boxcox

Review and clean up all Box-Cox functions