REF: partial extension to ContrastResults for t-test f_test, see #742
ENH: add summary_params_frame to ContrastResult and iolib.summary
REF: bring use_t back, partially, used in t_test
REF: use robustcov in RLM as default, see #1164
REF: add use_t in t_test, add wald_test with use_f
REF: allow title in summary_params, fix typo