It's not now clear what this is really for, except maybe for printing summaries of incomplete datasets.

It gets set if specified on creation -- or from.data if that is provided on initial creation --
but IIRC not otherwise.

save_errors makes no reference to it, but just checks ``.data`.

to_nc4 uses it to set the created variable type -- but this is probably a bug (#89)

So possibly, this has no actual function