OCSADV-27: Declination limits and Coordinates Difference

On this PR the limits for declination or changed to allow 90 degrees and CoordinatesDiff is rewritten in Scala using the new model classes

I wish I could do more thorough tests but I couldn't find the source of the algorithm nor some well known table of differences to compare
Right, okay that makes sense.  The other alternative to copying `CoordinateDiff` would be to translate the core `Angle` to/from skycalc `Angle` in AGS and then use `CoordinateDiff` directly  until it can fully be converted to the core classes.  That might keep you from having to stray too far into things that aren't directly related to the catalog work?

In one way or another, most all of skycalc is going to have to be replaced/updated to work with the core classes before it is over.  We've had (and still have) a lot duplication in the skycalc code so I'm a bit hesitant to introduce more unless there is a plan in place for removing it in the near future.  @tpolecat do you have any input on this?