ENH: fit_regularized for linear models

This is an implementation of fit_regularized using coordinate descent.  It allows "elastic net" regularization for OLS and GLS.  This includes the Lasso and ridge regression as special cases.

The tests include a number of comparisons to glmnet in R, the agreement is good.  Speed seems OK but I haven't done any timings.

Following recent discussions, I would like to rename 'alpha' to something more informative.  pen_wt or pen_wgt is my preference.  Whatever we choose needs to be done uniformly for all the existing fit_regularized implementations in order for solution_path to work.  If we can reach agreement I will put this into a separate PR.

The approach used here can also give us a fit_regularized for all GLMs and for the Cox proportional hazards model.  I'll get to this hopefully later in the summer.