InstData Import: pull out retrieve file methods into separate modules

This method seems to be named oddly as it doesn't retrieve a path, it just returns a boolean if both paths exist? If I'm reading this right, could the method body simply be

````perl
if ($self->_retrieve_metadata_path &&  $self->_retrieve_bam_path) {
    return 1;
} else {
    return;
}
````

Or maybe simplified even further to

````perl
return $self->_retrieve_metadata_path && $self->_retrieve_bam_path
````