This triggers bugs in the Rotation and RigidBodyMotion classes.

In particular, it triggers what Jonathan Youett fixed in the
never-merged merge request

  !2

These bugs are fixed in this commit, too:
* The RigidBodyMotion class gets a `log` method
* The Rotation<3>::log method now returns EmbeddedTangentVector
  instead of TangentVector.

Calling log(a,b) returned a-b instead of b-a.

Also, enable unit tests for the RealTuple class.  The current test
will not trigger the bug in the log method: that will come shortly.

Finally, this commit is the first that mentions its user-facing
changes in the newly introduced CHANGELOG.md file.

This involves fixing the secondDerivativeOfDistanceSquaredWRTSecondArgument
method: Previously it returned a FieldMatrix, but the test requires
a SymmetricMatrix.

Capturing any polymorphic object by value is wrong,
and gcc 8 started to warn about this in the case
of exceptions.

[[Imported from SVN: r9997]]

[[Imported from SVN: r9198]]

[[Imported from SVN: r9197]]

And use it in the test.

This patch uses the new 'constexpr' macro.  Hence it is C++11 only.

[[Imported from SVN: r9195]]

[[Imported from SVN: r9186]]

[[Imported from SVN: r8556]]

[[Imported from SVN: r8378]]

[[Imported from SVN: r8374]]

Which highlights the fact that the code was hacky in the first place:
These type conversion should not be necessary.  Will be fixed soon.

[[Imported from SVN: r8365]]

[[Imported from SVN: r8150]]

restrict testing to point that are closer than 0.5*M_PI apart.  That's a hacky way to avoid degenerate cases

[[Imported from SVN: r8038]]

[[Imported from SVN: r8036]]

[[Imported from SVN: r8035]]

[[Imported from SVN: r7367]]

[[Imported from SVN: r7343]]

[[Imported from SVN: r7309]]

[[Imported from SVN: r7308]]

[[Imported from SVN: r7307]]

[[Imported from SVN: r7305]]

[[Imported from SVN: r7304]]

[[Imported from SVN: r7303]]

[[Imported from SVN: r7201]]

[[Imported from SVN: r7136]]

[[Imported from SVN: r7067]]

[[Imported from SVN: r7046]]

[[Imported from SVN: r7045]]

[[Imported from SVN: r7041]]

[[Imported from SVN: r6435]]

[[Imported from SVN: r6434]]

Use the polarization identity from Absil, Mahony, Sepulchre (page 107) to compute an fd approximation of the Riemannian Hessian

[[Imported from SVN: r6431]]

[[Imported from SVN: r6429]]