creating theta for zerocorr models

[palday]

A few options

1. more documentation on how to do this directly (since theta is just the scaled standard deviations)
2. add in automatic conversion (with error checking) into update!
3. create zerocorr variants of create_re

I think (2) is the best given the current structuring of the package. If we added a create_theta method, it's less ideal because that method either needs the model as an argument or we need the RE covariance matrices to already be zerocorr'd.