Multivariable Regression

Multivariable Regression runs now. It doesn't get the right answer, though. I don't know if there's a bug in the initial simulation (`rmultivariableregression`) or in the regression (`lm.multivariable`). Maybe nothing's wrong and I just need to run it for more samples.

I have only exercised the flat prior case. I don't know if the other case works, though it looks correct to me.