gt output for marginal covariance model

[kyle-messier]

@ericbair-sciome @shail-choksi

I used gt and glue to create an example of a table output of the marginal covariance models. It's not too clean atm, but doing this for the marginal, cross models, and betas would be great.

[kyle-messier]

@ericbair-sciome @shail-choksi @brian-bk22 This was a quick and dirty demonstration. I might try another PR later, but you're welcome to use the bones here to update show or create a summary method. Closing for now.