add \ord operator for group orders

src/math/mathop/mathop.pysty

@@ -90,6 +90,7 @@ __END_OPTIONS_X__
 \DeclareSimpleMathOperator{Proj}
 \DeclareSimpleMathOperator{QCoh}
 \DeclareSimpleMathOperator{Fun}
+\DeclareSimpleMathOperator{ord}
 \DeclareMathOperator{\Nil}{\mathcal{N}il}
 \DeclareMathOperator{\Ouv}{\mathcal{O}uv}
 \DeclareMathOperator{\PreSh}{Pre-Sh}