flattened shallow copy of field

As discussed on Mattermost with @bauer a few days ago, the memory layout of Field<Vector3<real_t>, 1> is the same as that of Field<real_t,3> and it is thus possible to use them interchangeably by converting the metadata appropriately.

One minor thing I don't like about my current implementation: things like VectorTrait<std::string> don't produce an error anymore. You can't use it because it doesn't have the setter and getter and its OutputType is void, but still it doesn't feel right. My attempts to conditionally disable Field::flattenedShallowCopy failed, so that changing VectorTrait was my workaround.