[vary-obs-timetag] Issue #1146: Adding variable time tag specifications to range class,...

[vary-obs-timetag] Adding variable time tag specifications to range and angles observation classes, allow for obs to be specified at transit/bounce and tx/transmit. Retain current behaviour as default.

In the case of angles measurements an additional transmit case is included where the estimated value is calculated either from a transmit apparent or a receive apparent station position, whilst the time tag still describes the transmit time.