[Draft] add dtype to Info class

We currently only assume the data-type to be implicitly given with the data-array.

To store the data type explicitly we should add this attribute to the Info object as dtype (can follow numpy.dtype.str conventions).

We need to clarify the implications of this change and the default behavior.

Are different dtypes compatible? Do we need adapters?

What does the data-check do with different dtypes? (int vs. float and in-data vs. info)

mentioned in merge request !258 (merged)

Do we really want this? @mlange

Maybe we just keep the issue open and see whether there is any need for it later. So far, it seems not necessary.

changed title from [Enh] add dtype to Info class to [Draft] add dtype to Info class