Fixed missing transformation errors when obstime is None

[ayshih]

The transformations to/from SunPy 1.0 frames are fairly permissive when they encounter obstime=None for a frame and try to transfer it from the other frame in the transformation, or even the observer attribute if it's an observer-based frame. The behavior is no longer permissive if attempting to transform beyond that subgraph. (The permissive behavior was primarily motivated by our observer frame attribute, but the specific bug there has been fixed by #4266, and the concept of under-defined SkyCoord-less transformations may be going away in the future. This PR leaves this permissive behavior untouched.)

The transformations to/from SunPy 1.1 frames (HEE, GSE, GEI, HCI) don't fully handle all situations with obstime=None, and weren't raising transformation errors. Some of the transformation output in edge cases could be outright wrong.

This PR adds a bunch of transformation errors.

[Cadair]

I don't fully understand why we need these checks and not some others, or why the errors aren't already being raised in other places.

What is the difference between these transforms and the ones where we copy from the target frame?

[ayshih]

The idea is to always have the check when transforming between an Astropy frame and a SunPy frame, for when our frame has obstime=None, because copying may be weird. The check already exists for the HCRS<->HGS transformations, so we need analogous checks for the HME<->HEE and HME<->GEI transformations.

The HGS<->HCI checks are a bit different: it'll only error when obstime=None on both ends, so the transformations are not defined.

The HEE<->GSE behavior should actually be analogous to HGS<->HCI, so I'll fix that.

[ayshih]

Okay, HEE<->GSE transformations are now fixed. Plus, I realized that there were unnecessary computations in those transformations, so I also simplified them.