default comoving attribute of cosmo_array should be None?

[kyleaoman]

If we initialize a cosmo_array like this:

cosmo_array(np.array([1]) * u.Mpc)

the comoving attribute defaults to True. If we try to convert to physical units we get an exception because it tries to reference attributes of the cosmo_factor which is None, but trying to convert to comoving works fine. This seems dangerous, wouldn't it be better to default to None when the situation is ambiguous? Alternatively returning a unyt_array instead wouldn't be unreasonble, but is probably more confusing to someone expecting a comso_array. Could do it anyway and produce a warning.

Implementing this would need some checking that nothing in swiftsimio relies on the default. Test coverage might be good enough for issues to raise their head if it is relied on, but then again they might not ;)