Heckman

Added a module to estimate the Heckman selection model using the Heckman 2-step. A test module checks the parameter estimates that this module creates in an example dataset against the parameter estimates that Stata creates. If this happens to merge well into the existing statsmodels code, I'd like to add an MLE estimation method as well. Stata currently has the option to estimate the Heckman model by either the 2-step or by MLE.
Sorry, I realized my mistake while writing the general comments.
extra arrays in `__init__` are automatically handled together with endog and exog if they are given to the `super().__init__` as keywords, IIRC this time.

(The missing `missing` part is when we have additional arrays at a later stage as for example in `fit` for cluster robust standard errors.)