Preserve ndarray-like magnitudes or reject them from the start

[gerritholl]

I am trying to use a xarray.DataArray magnitude with a pint quantity. It appears to work, but doesn't really, because Quantity.__getattr__ forces it to convert to numpy.ndarray. This leads to conversion at very unexpected behaviour:

    In [526]: q = ureg.Quantity(xarray.DataArray([1, 2, 3]), 'm')
    
    In [527]: print(q)
    <xarray.DataArray (dim_0: 3)>array([1, 2, 3])Dimensions without coordinates: dim_0 meter
    
    In [528]: a = array(q)
    
    In [529]: print(q)
    [1 2 3] meter

I think pint should either accept magnitudes that behave like numpy.ndarray but don't subclass it (such as xarray.DataArray), or reject them when the object is constructed. I don't see how converting it in the middle of attribute lookup can do any good…

[hgrecco]

Revisit after #905

[jthielen]

I believe this should be resolved by #845, which is the more general issue about array-like compatibility, so I think this can be closed?

(For the example at hand, this should not be allowed, as xarray DataArrays should wrap pint Quantities and not the other way around.)