lib/src/*/Numeral*.gf: fix inconsistent inheritance form Cat

While the abstract Numeral inherits only Cat[Numeral,Digits], some of the
concrete NumeralNNN of Numeral inherited everything from CatNNN.

Normally the compiler outputs a warning message when a concrete syntax 
contains superflous lincats, but apparently not when they get included
through inheritance...

This does not seem to cause problems in the Haskell run-time system, but the 
C run-time system fails to load PGFs with superflous lincats.

This problems shows up when creating application grammars that inherit Numeral.
The Phrasebook is an example of such a grammar.

lib/src/romanian/NumeralRon.gf

@@ -1,4 +1,4 @@
-concrete NumeralRon of Numeral = CatRon ** 
+concrete NumeralRon of Numeral = CatRon [Numeral,Digits] **
   open  MorphoRon, CatRon, Prelude in {
 
 flags coding = utf8 ;