Wald anova

wald test for terms that go over several columns

basic version tested for OLS, GLM and Poisson,  use_t=False only
It's not an ANOVA table, we ignore sum of squares or equivalent.

not clear: name of method, currently is `wald_anova`, alternative `wald_test_terms`  ???

**missing**

- unit tests for use_t=True, i.e. F instead of chi2
- handling of extra parameters, as in NegativeBinomial (not checked yet)
- improvements to test results class
- unit tests for cases when no formula is available

- ignored for now: Multinomial where several things don't work

**extension**

- multi-equation models as in BetaRegression, and others to come

**open**

should we add `wald_null` ?  
hypothesis: all slope coefficients are zero, only constant is non-zero