fix typo

disambiguation dilemma between "sauce" and "source"
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
ENH: GLS fix for very large models

GLS can now handle very large models with diagonal sigma matrices.  This was done by removing the need for a full nobs by nobs covariance when sigma is input as a scalar or vector.

Test coverage for large model with 100,000 observations.  This would require > 64GB of main memory in previous GLS code.
Simplified branching in GLS.whiten

Removed one level of an if statement in GLS.whiten and reordered
from lowest to highest for the dimension
TST GLS_large_data lower samplesize (no memory test anymore), fix typo
Merge branch 'gls-large-data-size' closes #1176