Support for `emp_start` to apply to other non-spatial covariance structures

Summary

The current function emp_start works well to speed up us covariance estimation. I would like to extend this to the other non-spatial covariance structures so that we start with a close approximation before going into a general optimization loop.

Additional Information

Essentially, the existing emp_start need only use an additional helper function that calculates a reasonable starting value for non-us covariance structures from the empirical covariance matrix.

Start at the existing emp_start entry point and trace how it derives starting values for us covariance estimation. Then inspect the other non-spatial covariance structures and determine the reasonable empirical-covariance starting values needed for each. Done means emp_start provides useful starts for those structures before the general optimization loop.