Overhauled PVTCoord to represent a Coord with instantaneous velocity
instead of velocity along a great circle.
PVTCoord improvements and getDist->getDistance

Improved the PVTCoord constructors.
Added PVTCoord.getDistance method.
Bug fix: PVTCoord.offset was not setting toOrient.
Improved PVTCoord unit tests, including enabling offset test
and constructor with vector vel test.
Added an isCurrent flag to Coord based on date=0

Date=0 is now the default for most coordinates.
Added a tai argument to the Coord version of makeCoord;
this is used to set the date of (a copy of) the from or to coordSys
if that coordSys is current (has a date of 0).
Modified so FK5 and FK4 date=0 implies 2000 and 1950, respectively
Modified so that a date of 0 is disallowed for FK4CoorSys and FK5CoordSys
Modified a test to use distanceFromParallax