Initialization of result in NDData methods _arithmetic and convert_unit_to prevents subclasses from requiring units

[mwcraig]

Briefly, the NDData._arithmetic currently initializes its result with

        result = self.__class__(data)  # in case we are dealing with an inherited type

and then goes on to set result.unit, etc. That causes an issue for subclasses that want to require units.

A PR that fixes this (and a similar issue in NDData.convert_unit_to) is on its way.