Create simdf from a dat file

[kellijohnson-NOAA]

The general setup_scenarios_defaults() is based on a random simulation. Instead, users sometimes want the baseline sampling to come from a file. It would be great if we could create simdf from an r4ss::SS_readdat() list, e.g., read in the OM data file using {r4ss} and parse out the data types that are available into the simdf structure. Thanks to @Chang-Zhang for bringing this up and walking me through an example.