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
void, but still it doesn't feel right. My attempts to conditionally disable
Field::flattenedShallowCopy failed, so that changing
VectorTrait was my workaround.