BUG: fix ttest_power for 2-sided 2 rejection areas
REF: CompareMeans now ttest and confint are independent of ddof
DOC/REF: add new stats to stats/api.py and stats.rst docs
REF: some cleanup, rename diff -> value in ttest_ind argument
STY: use "Mixin" to avoid pylint:E1101
BUG/REF: fixes after renaming diff -> value in ttest_ind (tests in test_tost.py)
REF/TST add default value=0 for one sample ttest_mean;
        add tests for no effect of ddof in one sample ttest_mean, confint_mean
TST: add first test for NormalIndPower and normal_power
REF/TST NormalIndPower allow ratio=0 for one-sample, test classes with root testing
TST: more test cases for ttest power
BUG/TST: test and fix rootfinding for GofChisquarePower
REF/DOC: use power method instead of solving for beta,
         DOCstrings typos, corrections
TST/REF: tests for chisquare, force chisquare to be non-negative with correction
ENH: ttest power, allow smaller/larger as one-sided tests, docstrings not changed
ENH: NormalIndPower enable larger/smaller one-sided tests
REF: change one-sided option to larger/smaller, change docstrings
REF: rename beta -> power, and fix example
ENH: ftest_power (2 versions) with tests, no class yet
ENH: classes for Ftest power, 3 precision test failures
REF/TST: add start_value as class attribute, overwrite in Ftest,
         rootfinding error in FtestAnova for effect size -> special case it
         refactor unit test for lower or relative precision