Initial addition of heteroskedasticity robust LM and LR tests.
* Initial addition of heteroskedasticity robust LM and LR tests.
* Additional Clean up
Bug fixes for lr test
fixed typo
Added test that check whether OLS, GLS and WLS produce the same results when the weight matrix or sigma are diagonal with the same values
Added basic tests for LM
LM test now handles robust covariance
Fixed loglikelihood in WLS
Lots of Typo fixed in docstrings
PEP8 fixes
Added _is_nested method to detect nested models
rebase conflicts fixed, hopefully
Added tests for compare_lm_test
Fixed test for summary which was failing due to now mandatory cov_type
Added additional OLS/WLS/GLS equivalence tests
Added _has_constant function which can detect implicit constant
Added _is_nested to test whether models are nested when comparing models
Added compare_lm_test
Merged with get_robustvcv_result which is needed to implement tests
Added large sample likelihood-ratio test
Implemented cov_type for all models including homoskedastic regression.  Use 'H' for the homoskedastic case
heavily edited for 'nonrobust' instead of 'H' during rebase
ENH: Added method _has_constant which can correctly detect an implicit constant
ENH: Normalization of weights in WLS and loglike correction
ENH: Added cov_type for all models
Made tests tougher
BUG: fix rebase error
Merge pull request #1214 from josef-pkt/compare_lr_test_rebased

Compare lr test rebased closes #1174