fix: add make_field_function (#266)

## Description
The `make_field_function` was removed in a previous version, but it is
used in external projects. This PR simply adds it back.

## Code of Conduct
By submitting this PR, you agree to follow our [Code of
Conduct](https://github.com/hpc-maths/samurai/blob/master/docs/CODE_OF_CONDUCT.md)
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include/samurai/field_expression.hpp

@@ -122,6 +122,13 @@ namespace samurai
         , m_f(std::forward<Func>(f))
     {
     }
+
+    template <class F, class... E>
+    inline auto make_field_function(E&&... e) noexcept
+    {
+        using type = field_function<F, E...>;
+        return type(F(), std::forward<E>(e)...);
+    }
 } // namespace samurai
 
 namespace xt::detail