AsgTools: Fix clang compilation issues.
In AsgToolHandle, we have overloads like
template <class T2> StatusCode
setProperty (const std::string& property, const T2& value);
template <class T2> StatusCode
setProperty (const std::string& property, const ToolHandle<T2>& value);
However, ToolHandle has a constructor which can accept an integer. When called like
tool.setProperty<int> ("propertyInt", 42)
some versions of clang were using the second overload rather than the first.
Use enable_if to ensure that the ToolHandle/ToolHandleArray overloads are not used for types which can't be tools.