dxtbx_ext => dxtbx.__init__

[Anthchirp]

We currently

from dxtbx_ext import *

in top level dxtbx.__init__

Suggest moving this to dxtbx.ext with a one-release grace period with deprecation warnings.

This would

• avoid polluting the dxtbx.* namespace, and therefore
• make clear when extension methods are used, and
• skip the automatic import of the extensions on importing dxtbx, which may not always be required

Opinions?

[ndevenish]

+1. Extension imports should only happen in their own module where they can be handled/patched. I wouldn't be surprised if there were root dxtbx names used that this moves, but it's better to have this separate even if we end up having to just explicitly import everything back into the dxtbx namespace.

[Anthchirp]

candidate patch added