Merging spreg's repo with pysal 1.6. 

Merging spreg's repo with pysal 1.6.

Change log:
- Adding group-wise heteroskedasticity correction to OLS_regimes;
- Adding optimal GMM estimator to account for heteroskedasticity in TSLS_regimes;
- Adding several tests for regimes methods;
- Adopting sparse matrices for base functions;
- Restricting parallel to operations to non-Windows systems;
- Setting White test as optional in OLS models;
- Updating results printouts;
- Fixing several minor bugs;
- Fixing documentation of several methods.