ΣMeasurement used to extract types of fields of Measurement to define
same fields of itself. However with upcoming Stat and StatT "generic"
types, they can be used with several underlying dtypes, e.g.
Stat(dtype=int32) and Stat(dtype=float64), so using just a type class
when copying definition of a field will loose information.

-> Fix that by always using dtype of a field as definitive source of
   what it is.

NOTE that dtype has .type attribute which refers to attached type class
that represents scalars of that dtype.