Update documentation for ctr and uncTime.

ctr has new documentation and restrictions: Barycentric centers are not permitted. Planetary and Solar System barycenter values (SPICE IDs 0-9) will not validate. New documentation for ctr states,

Origin of the reference system given by sys using SPICE IDs. (See http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/naif_ids.html.) To avoid model dependencies, barycentric positions for ctr are disallowed. This is enforced by the ADES schema for planetary and solar system barycenters, though the use of minor planet system barycenters is also not compliant with ADES. The use of values other than ctr=399 are generally discouraged and require advance consultation with the MPC. Note that sys=WGS84 implies ctr=399.

Following lengthy discussion with Micheli, uncTime also has new documentation that slightly enlarges its use and interpretation:

Estimated systematic time error in seconds. Unlike the preceding RMS fields, which indicate random errors, this field indicates a presumed level of systematic clock error. This element can be used to communicate the quality of a clock calibration if one was conducted, in which case it would represent the RMS of the systematic error in obsTime. In unusual cases, it can also be used to communicate the size of a large and unknown clock error that is markedly different from an observatory’s usual performance. If no calibration has been conducted then the element should be neglected.

IAU-ADES / ADES-Master

Update documentation for ctr and uncTime. #55