mibios/query: add only(), select_related() methods to ChainedQuerySet

This fixes a regression, export of sample=-based taxabundance data works
again.

mibios/query.py

@@ -1146,9 +1146,15 @@ def iterate(self, **kwargs):
     def values_list(self, *args, **kwargs):
         return self._apply_queryset_method('values_list', *args, **kwargs)
 
+    def only(self, *fields):
+        return self._apply_queryset_method('only', *fields)
+
     def prefetch_related(self, *args, **kwargs):
         return self._apply_queryset_method('prefetch_related', *args, **kwargs)
 
+    def select_related(self, *fields):
+        return self._apply_queryset_method('select_related', *fields)
+
     def count(self):
         """
         Not implemented but declared here to make class pass django_table2's