CDF_TIME_TT2000 should not include leap seconds

[maxinelasp]

The documentation in cdflib.epochs.py states that CDF_TIME_TT2000 includes leap seconds. However, per this document, CDF_TIME_TT2000 is in TT format and therefore does not add leap seconds.

From SDFC: "Given a current list of leap seconds, conversion between TT and UTC is straightforward (TT = TAI + 32.184s; TT = UTC + deltaAT + 32.184s, where deltaAT is the sum of the leap seconds since 1960; for example, for 2009, deltaAT = 34s). "

Per Wikipedia and NAIF, TT is based on TAI which does not add leap seconds.

Therefore, I believe this documentation is wrong. However, from looking through the code, in particular breakdown_tt2000 which converts TT2000 into UTC and correctly includes a leap second addition, this is only an issue in the documentation and not in the code itself.

If this is an issue, I'm happy to open a PR to correct it - just wanted to check in and see if I am right first.