add accessor to Celestial
Fix constness in Barycentre
Implement barycentric corotating frame
Fix constness in Barycentre
Merge branch 'body-accessor' into barycentric-rendering
Merge branch 'fix-constness' into barycentric-rendering
how will it fail?
Sort of works
Revert "Fix constness in Barycentre"

This reverts commit e6f99ad8f5112b5d2c978cc59f0f763eca420d77.
Merge plot-trajectories after pleroy's review.
Merge branch 'plot-trajectories' into barycentric-rendering
mergez la vache!
After pleroy's review
Merge pull request #250 from eggrobin/barycentric-rendering

Barycentric rendering