ssm_data_prep: Prepare data for a state-space SRA fit
In bstaton1/FitSR: Functions to Fit and Summarize SRA Models

Takes the output of the SimFit functions to prepare for fitting a state-space spawner recruit model

1	ssm_data_prep(params, obs, covariance)

`params`	a list object created by `SimFit::init_sim()` or `kusko_data_prep()$params`. Contains the driving parameters and dimensional variables.
`obs`	a list object created by the functions that generate observations in the `SimFit` package or the `kusko_data_prep()$obs` function.
`covariance`	a character vector of length 1: `"simple"` or `"complex"`