hasattr(obj, beam) causes exception

[keflavich]

Regression introduced by #700, I believe:

ipdb> hasattr(obj,'beam')
*** spectral_cube.utils.NoBeamError: No beam is defined for this SpectralCube or the beam information could not be parsed from the header. A `~radio_beam.Beam` object can be added using `cube.with_beam`.

This is because you can't access a .beam attribute without raising an exception if it's not defined. But, the object will always have the .beam attribute.

[keflavich]

I have a fix in progress

[e-koch]

Fixed in #720