Disambiguating on-disk dtype and effective dtype

[effigies]

There is often confusion about the meaning of img.header.get_data_dtype() even from regular users of nibabel. I would propose having an explicit concept of raw (on-disk) and effective dtypes (provisionally, rdtype and edtype).

class DataobjHeader:
    @property
    def rdtype(self):
        """Raw type of data object, as stored on disk"""

    @rdtype.setter
    def rdtype(self, dt):
        """Set the type that data should be coerced to when saved to disk"""

    @property
    def edtype(self):
        """Effective type of data object, when scaled"""

    @edtype.setter
    def edtype(self, dt):
        """Set the type that data should be coerced to when read into a numpy array"""

This would allow for checks like:

if np.issubdtype(img.header.edtype, np.integral):
    # Probably a label or mask file
else:
    # Probably not intended to be discrete

Currently the easiest way to find out the effective dtype is img.dataobj[0, 0, 0].dtype.