## Function `epochProp( tYr, astrom6 )`

Description:
  Propagates the astrometry parameters, supplied as a 6-element array, to a different epoch.

  The input and output astrometry parameters are each represented by a 6-element array, with the following elements:

       index  gaia_source name  unit    description
       -----  ----------------  ----    -----------
         0:   ra                deg     right ascension
         1:   dec               deg     declination
         2:   parallax          mas     parallax
         3:   pmra              mas/yr  proper motion in ra * cos(dec)
         4:   pmdec             mas/yr  proper motion in dec
         5:   radial_velocity   km/s    barycentric radial velocity

  The units used by this function are the units used in the `gaia_source` table.

  Null values for `parallax`, `pmra`, `pmdec` and `radial_velocity` are treated as if zero for the purposes of propagation. The documentation of the equivalent function in the Gaia archive comments "This is a reasonable choice for most stars because those quantities would be either small (parallax and proper motion) or irrelevant (radial velocity). However, this is not true for stars very close to the Sun, where appropriate values need to be taken from the literature (e.g. average velocity field in the solar neighbourhood)."

  The effect is that the output represents the best estimates available for propagated astrometry; proper motions, parallax and RV are applied if present, but if not the output values are calculated or simply copied across as if those quantities were zero.

Parameters:
  `tYr` (floating point)
    epoch difference in years
  `astrom6` (array of floating point)
    astrometry at time t0, represented by a 6-element array as above (a 5-element array is also permitted where radial velocity is zero or unknown)

Return Value (array of floating point):
  astrometry at time `t0+tYr`, represented by a 6-element array as above

Example:
  `epochProp(-15.5, array(ra,dec,parallax,pmra,pmdec,radial_velocity))` - calculates the astrometry at 2000.0 of gaia_source values that were observed at 2015.5

Signature:
  `double[] epochProp(double, double[])`