First efforts at internilizing change_bin
Change call to r4ss functions in sample_mlacomp.R

Using importFrom instead of :: to reference functions in the r4ss package.
Add check to sample_mlacomp.R to make sure that mla comps exists.

Both that the mean size-at-age data exists and that AgeErr > 1
indicating mean length-at-age data.
Fix spelling in sample_mlacomp.R
Fix test in sample_mlacomp that failed if list was of length one.

If only one year R does not recognize it as a list it is a numeric, even
though it is specified as a list.
Changed mwa_params = NULL because currently there is no sampling routine