Absolute error.
Relative error.
Undo test changes.
Merge pull request #226 from pleroy/Norm

Simplify the implementation of absolute and relative errors by using pointers-to-function for the norm
add accessor to Celestial
Fix constness in Barycentre
Implement barycentric corotating frame
Fix constness in Barycentre
add accessor to Celestial
Merge branch 'body-accessor' into barycentric-rendering
Merge branch 'fix-constness' into barycentric-rendering