Data classes should be fussier about typing

[pp-mo]

We should maybe make more effort to enforce correctness
(whereas up to now we have just aimed to "not make it hard to do the right thing")

Probably we should disallow, e.g. variable.attributes = {'a':1, 'b':2}
(which currently is a gotcha because the content is just values, not NcAttributes)
Or, we should support the correct translations