[forms][bugfix] Fix GeometryType in QuadratureRuleKey exported by Forms::Coefficient

So far a `Forms::Coefficient` exported a `QuadratureRuleKey` with
`GeometryType` being `none` with appropriate dimension. This works
when assembling linear and bilinear forms, because we always multiply
with at least one unary operator which then sets the correct `GeometryType`.

However, if we want to use the nullary operator `Forms::Coefficient`
standalone, e.g. to simply integrate it (it's a grid function after
all), we silently use the wrong `GeometryType`. Luckily we can always
derive the correct `GeometryType` from the element this function is
bound to allowing for an easy fix.

dune/fufem/forms/coefficient.hh

@@ -65,7 +65,7 @@ namespace Dune::Fufem::Forms {
 
       auto quadratureRuleKey() const
       {
-        return QuadratureRuleKey(Element::dimension, order_);
+        return QuadratureRuleKey(LocalFunction::localContext().type(), order_);
       }
 
       template<class... LV>