Support Arithmetic Operations between NDCube and WCS-less NDData

[DanRyanIrish]

Describe the feature

Currently, NDCube does not support arithmetic operations with NDData instances because it does not make sense to combine cubes on different coordinate grids. However, this is not a concern when the NDData instance has a wcs set to None. Therefore, we should support arithmetic operations between NDCube and WCS-less NDData instances. This would serve the cases where users want to NDCube with data with uncertainties and a mask.

Proposed solution

This would need to be explicitly supported in NDCube.__add__ and NDCube.__mul__.

For adding, code needs to be inserted here that checks if not hasattr(value, wcs) or value.wcs is None: and then add then convert the value to the right units ( u.Quantity(value.data, unit=value.unit).to_value(self.unit) ), add the data as masked_arrays, (new_data = np.ma.masked_array(self.data, mask=self.mask) + np.ma.masked_array(value.data, mask=value.mask), and propagate the uncertainties, An example can be seen in NDCube.__pow__ for multipy uncertainty propagation. Also, see astropy.nddata.NDUncertainty.propagate for more info.

A similar structure should work for multiplication somewhere in the codebase around here.