Remove Volume.arraydata because it does not round-trip to the correct Volume shape

[ylep]

The Volume.arraydata() function is used by many scripts, both in our distributed code and in user scripts. However, this is the incorrect function to use, people should use numpy.asarray or volume.np instead.

Scripts that use Volume -> arraydata() -> NumPy Array -> Volume used to work by sheer chance because of a bug in aims.Volume, but that bug is now fixed and thus user scripts needs to be fixed (see #92 for details).

I propose to drop support for arraydata() entirely, making it raise an exception telling people to fix their code and use numpy.asarray or volume.np instead. If some specific use-cases need its functionality, we can make it available under another name (e.g. raw_internal_array or something similar).

We also need to fix the code that we distribute: I counted 487 uses of arraydata in Python code in the cea distro on the 5.1 branch:

$ ack -ch --python --ignore-dir=5.0 --ignore-dir=master '\barraydata\b'
487

[ylep]

OK I agree to make arraydata() obsolete.
The method also exists in vectors and textures (1D arrays), and is harmless there and identical to asarray(), so there are certainly less than 501 "dangerous" uses of arraydata() on volumes, but I also agree that we should deprecate it there too.

Originally posted by @denisri in #92 (comment)