ENH: implemented Heckman 2 step
TST: added test for Heckman 2 step
DOC: updated comments
DOC: added new heckman module to release notes
ENH: heckman summary now includes parameters from both 2-step equations
STY: cleaned up heckman test code
STY: cleaned up heckman description in release notes
STY: broke long line into multiple lines
STY: renamed select_exog to exog_select, moved fitting routines into private methods, renamed fitted to results
DOC: removed Heckman addition from 0.6 release notes since it should be for 0.7
DOC: added clarification to data format that the Heckman module expects
TST: Heckman test now loads internet sourced data using pandas
ENH: added missing values checking and let inherited LikelihoodModels routines do more
TST: fixed warning in test from doing pandas replacement on a view
ENH: added predict method
TST: added test for predict method
ENH: added log likelihood function for heckman
ENH: added Heckman estimation by MLE using numerical derivative methods
BUG: LikelihoodModelResults.conf_int use df_resid_inference closes #1903
Add option to set covariance clipping method, improve reporting of linalg errors