CECoordinates conversion routines too confusing

[Jvinniec]

Simply put, there are too many ways to do coordinate conversions in CECoordinates. These should be simplified to reduce the confusion on exactly how a user should transform between coordinates. The methods should be rewritten to have the following properties:

• [x] Take only CppEphem objects: They should use CECoordinates, CEDate, CEObserver classes to do to the transformations. This means dropping all the methods that allow passing/returning angles as doubles. It will also facilitate the next point.
• [x] One entry point for a given conversion: For example, to convert to observed coordinates there is currently ConvertTo, ConvertToObserved, GetObservedCoords (short form), GetObservedCoords (long form), and 6 additional methods accounting for a short/long form for each of the XXXToObserved methods. I would propose: ConvertTo, ConvertToObserved, and a single XXXToObserved method. This drops the number of methods from 10 to 5.
• [x] Remove all references to dut1 and x,y polar motions: PR #15 basically resolves passing correction terms and they are now handled internally and the user doesn't have to concern themselves with it.

If there are no objections, I will implement these changes for v1.3.

[Jvinniec]

This change can also be extended to CECoordinates::AngularSeparation methods (there are 3). Doing so would resolve the confusion when comparing observed coordinates and also reduce the number of methods from 3 to 2.

[Jvinniec]

Resolved via PR #28