update ecx and nsec to always return a tibble

[beckyfisher]

At the moment a different type of object is returned depending on the method, if there is a grouping variable and if the posterior is being returned, or summary statistics. I suggest we updated all so they consistently return a tibble, even when the posterior = TRUE, in which case it would return a nested tibble. This will allow consistency, and also ensure multiple ecx_val values can be seamlessly handled.