SFINAE.
Fix client
Revert "Fix client"

This reverts commit c27d06624b47b3205098704ebe8bdd1c4e2d8b08.
New files and abstract Body.
Massless and some Trajectory changes.
Massive and oblate.
Painfully fix clients.
More clients, more pain.
Even more body pain.
The mess with Celestial construction.
Compile and fix tests.
Merge branch 'master' into BodyInertial
Performance.
Attempt at checking that frames match.
After egg's review.
Merge pull request #296 from pleroy/BodyInertial

I sing the body inertial.