The ProductManifold class generalizes RigidBodyMotion, and can do
everything that the RigidBodyMotion class can.  Therefore there is
no point in keeping RigidBodyMotion any longer.  Having two
implementations for the same thing will just confuse people.

In particular:
* Forward declaring ProductManifold in the `impl` namespace
  is not a good idea.
* Add missing headers

targetspacetest.cc is extended to test for the first issue.

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]]