Standards of Fundamental Astronomy - Issue: 2017-04-20 Changes

The changes fall into the following categories:

Implementation of the IAU 2012 definition of the astronomical unit (au). Not only has this constant been updated, but also
- all derived quantities are evaluated via the appropriate formulas, and
- all documentation now uses the appropriate symbols for the unit, for example, 1 au and 2.3 au/day.
Update to PVSTAR to make it consistent with the version of STARPV that was updated in the previous release.
Other minor typographical corrections to various routines.

The IAU 2012 value of the astronomical unit (au) is now implemented in these six routines:

An expression (sqrt(1-x)-1), where x is usually tiny, has been replaced with (-x/(sqrt(1-x)+1) to improve numerical precision in iau_PVSTAR.
Check values in the validation program t_sofa_f have been updated due to the change in the astronomical unit.
The SOFA Astrometry Tools cookbook sofa_ast_f.pdf:
- In the table on page 22 there is a change of +1×10^-7 seconds in RA for three results when compared to using the previous value for the au. These cases are repeated on the next 2 pages.
- The listing of the relativistic version of the routine iau_PMPX in the Appendix has been updated on page 77 with the required changes for the au.

The astronomical unit (au) has been updated in the sofam.h include file and all constants that are derived from the au have been replaced by the appropriate formulas.
An expression (sqrt(1-x)-1), where x is usually tiny, has been replaced with (-x/(sqrt(1-x)+1) to improve numerical precision in iauPvstar.
Check values in the validation program t_sofa_c have been updated due to the change in the astronomical unit.
The SOFA Astrometry Tools cookbook sofa_ast_c.pdf:
- In the table on page 20 there is a change of +1×10^-7 seconds in RA for three results when compared to using the previous value for the au. These cases are repeated on the next three pages.