clean the initial model files

Usage

clean_init_mod_files(
  OM_out_dir,
  EM_out_dir = NULL,
  MS = "EM",
  overwrite = FALSE
)

OM_out_dir: The full path to the directory in which the OM is run.
EM_out_dir: Relative or absolute path to the estimation model, if using a model outside of the SSMSE package.
MS: The management strategy to use. Current options are: "last_yr_catch" which uses the previous year's catch; "no_catch" which uses 0 catch; "EM" which uses an stock synthesis model as the estimation method and the management strategy as defined in the forecast file of the stock synthesis estimation method; "Interim" to modify catch based on survey predictions between assessments. Users can also specify their own management strategies as a function. For example, if the function is called "my_ms" then the user should specify MS = "my_ms" and specify the path to the file containing the function in custom_MS_source.
overwrite: Allow existing files to be overwritten?