What does zoom level mean when viewer has no concept of pixels?

[pllim]

Currently zoom level is defined in pixel space. However there might be situations where pixel space is meaningless in a viewer. Examples (non-exhaustive):

1. Imviz when linked by WCS uses a fake 10x10 hidden image as reference WCS. This causes zoom level value calculated in pixel space to be useless.
2. Sky viewer like WWT or Aladdin that has no concept of pixel space, only the sky.

What should we do:

• Allow these viewers to return None?
• Allow zoom level to be calculated in sky (arcsec, arcmin, arcdeg)? How?

[mwcraig]

This is fixed by switching to a viewport which has a field of view. In #46 zoom is completely eliminated.