Small JulianDate cleanups.

* Use a scratch variable for leap second lookups instead of allocating.
* Add an instance compareTo method, use that from LeapSecond.compareLeapSecondDate
Fix a number of places that were using reference equality on JulianDate instead of equals.
Fix inconsistent name of cache variable
Merge pull request #558 from AnalyticalGraphicsInc/fixDateComparisons

Fix date comparisons to use equals