More orbit APIs

Here's the pull request I promised you. The included changes are:

* Added t.universalTime API
* Added timeOfPeriapsisPassage to all orbit APIs
* Added a date unit type for the above APIs
* Added trueAnomaly to all orbit APIs
* Fixed the mean anomaly at epoch orbit APIs to correct for the orbit's epoch relative to universal time
* Increased the precision of telemetry data to 6 significant figures (so small values like eccentricity, mean anomaly at epoch, inclination, etc have enough precision to be useful)