LinearModel not setting data

[pdb5627]

When I originally worked on LinearModel, I assumed that fit() would set data after the model was built, so build_model just put in some dummy data. In later refactoring, this assumption turned out not to be true, so LinearModel never set the model data in the fitting process. I will prepare a PR for a test to check the fit of parameters and add setting data to LinearModel.build_model.