Generalize (deprecate?) get_xyzt_units

[effigies]

Nifti1Header.get_xyzt_units is entirely based on the bitfield as it is implemented in nifti1.h. Attempting to generalize it to Minc (#1098) requires reducing the available information, specifically raising a ValueError if the spatial axes have different units.

I think it makes sense to keep this interface which will be mostly useful, but we could start by generalizing, e.g.,

class HasUnits:
    def get_units(self)
        ...

And keep the old behavior:

    def get_xyzt_units(self):
        all_units = self.get_units()
        spatial_units = ...
        temporal_units = ...
        if len(set(spatial_units)) > 1:
            raise ValueError
        if len(set(temporal_units)) > 1:
            raise ValueError
        return spatial_units[0], temporal_units[0]

OTOH, maybe with get_units(), get_xyzt_units() is redundant and can be deprecated.

Thoughts?

[matthew-brett]

It's attractive to have only one interface to get the units - is the downside just the usual costs of deprecation?

[effigies]

It's more that the upside of having get_xyzt_units() is that you might want a single spatial unit and allow it to raise an exception if it can't be satisfied by the file. Having that check implemented in a single place reduces the odds that a caller gets the check wrong or two different callers might choose different exception types for the same problem.