Attempt to calculate delta_ut1_utc when the attribute is accessed directly
More explicit setting of jd1,jd2 in _get_delta_ut1_utc
Add bug-fix entry to CHANGES.rst
Correct UT in calculation of TDB; closes #1938
Also update TDB, TCB in test done in documentation
Merge pull request #1939 from mhvk/time-correct-ut-in-tdb

Correct UT in calculation of TDB; closes #1938
Merge pull request #1925 from mhvk/time-delta-ut1-utc

Attempt to calculate delta_ut1_utc when the attribute is accessed directly