Use sample_mlacomp to generate data for external growth estimation

The external estimate now reads in a csv file created when the argument
mean_outfile is non NULL. The csv file is placed in the em folder and
later referenced by change_e if the vbgf is called.
Fix bug in ss3simbase.r when data type is NULL

Now ss3sim_base references both the fleets and years to see if the
data type should be NULL.