Load by delegating to sub-loaders.

def load_adjusted_array(self, domain, columns, dates, sids, mask):
        """
        Load by delegating to sub-loaders.
        """
        out = {}
        for col in columns:
            try:
                loader = self._loaders.get(col)
                if loader is None:
                    loader = self._loaders[col.unspecialize()]
            except KeyError:
                raise ValueError("Couldn't find loader for %s" % col)
            out.update(
                loader.load_adjusted_array(domain, [col], dates, sids, mask)
            )
        return out