Add option to set covariance clipping method, improve reporting of linalg errors
Improvements to params_sensitivity
Don't alter the user's model instance
Incorrect initialization of cov_adjust
Fix #1877 relating to missing data handling in GEE
Don't test for missing with formulas
Add failing test t_est_missing_formula
REF/ENH Merge branch 'gee_sensitivity3_rebased' closes #1840