Alternate approach to missing location-related metadata

This PR is an alternate approach to #1045 when a map does not contain location-related metadata (Sun-spacecraft separation or apparent angular radius of the Sun).  Instead of returning a constant value, as currently done, or `None`, as #1045 does, this PR returns the appropriate values derived for the date/time of the map assuming an Earth-based observation (e.g., LEO), and explicitly says what it is doing.  This assumption is invalid only for those non-ground/LEO observers crazily fail to include such metadata.

A message is printed (using `print`) every time such a property is accessed.  I think using `warning` may be too easily ignored.
this is effectively equivalent to `from sunpy import sun` in 99% of cases.