Pygaia

* Test cases slightly updated

* Added constant AU expressed in mas*km*yr/s
* Fixed large memoery consumption in the _getJacobian method in the coordinate transformation code.
Thanks to [jchelly](https://github.com/jchelly) for pointing out this problem.

* Add method to CoordinateTransformation for the transformation of the full (5x5) covariance matrix of
the astrometric parameters.
* Add keyword to astrometric errors prediction functions that allows to specify an extended mission
lifetime.

* Bug fix in coordinate transformation code.
* Updated photometry and radial velocity error predictions.
* End-of-mission photometry errors, including calibration floor, introduced for the broad band fluxes.
* Photometry errors now include 20% margin for CCD-transit and end-of-mission predictions.
* Example plot of photometry errors fixed.

* Added numerical constants.
* Improved setup.py, made code compatible with Python3

* Minor error in documentation of astrometryToPhaseSpace() method corrected.