Add FiniteElementCollection::GetTraceFE

This is a variable-order version of TraceFiniteElementForGeometry, analogous to
FiniteElementCollection::GetFE, which is a variable-order version of
FiniteElementForGeometry.

fem/fe_coll.hpp

@@ -195,6 +195,17 @@ class FiniteElementCollection
       return var_orders[p]->FiniteElementForGeometry(geom);
    }
 
+   /// Variable order version of TraceFiniteElementForGeometry().
+   /** The order parameter @a p represents the order of the highest-dimensional
+       FiniteElement%s the fixed-order collection we want to query. In general,
+       this order is different from the order of the returned FiniteElement. */
+   const FiniteElement *GetTraceFE(Geometry::Type geom, int p) const
+   {
+      if (p == base_p) { return TraceFiniteElementForGeometry(geom); }
+      if (p >= var_orders.Size() || !var_orders[p]) { InitVarOrder(p); }
+      return var_orders[p]->TraceFiniteElementForGeometry(geom);
+   }
+
    /// Variable order version of DofForGeometry().
    /** The order parameter @a p represents the order of the highest-dimensional
        FiniteElement%s the fixed-order collection we want to query. In general,