Add C++ visualization API and some macros for scalar contexts

[ManifoldFR]

This pull request upstreams the contents of the single-header library Simple-Robotics/pinocchio-visualizers, as discussed on issue #2573.
This API reproduces the behaviour of the existing Pinocchio Python bindings' pinocchio.visualizers.BaseVisualizer class (which is in Python).

• Add header <pinocchio/visualizers/base-visualizer.hpp>
• Add new CMake target pinocchio_visualizers
• Add namespace pinocchio::visualizers
• Add base class pinocchio::visualizers::BaseVisualizer, for the context::Scalar scalar type. Common functionality (updating geometry placements, handling frames, the viz.play(qs) function) is compiled into the new pinocchio_visualizers target.
• Add pinocchio::python::VisualizerPythonVisitor visitor for exposing visitor classes

Resolves #2573

Lingering questions

• As a way of forcing implementers to implement the right functions, should we:
  • Keep using a virtual class with pure virtual members?
  • Or just have the API throw a runtime error if unimplemented members get called?
  • Or expose a CRTP class with the actual common logic implemented in a private base class?
• Better name or mechanism for the protected virtual member function BaseVisualizer::displayPrecall() - the idea is to be able to inject a hook before the kinematics and geometry get updated, e.g. locking the GeometryData using a mutex.
• Whether to name all of the unimplemented functions XXXImpl() or XXX_impl() and whether to make them protected.
• How to fuse this into the actual Python BaseVisualizer class, which we could significantly cut down.

[github-actions]

👋 Hi,
This is a reminder message to assign an extra build label to this Pull Request if needed.
By default, this PR will be build with minimal build options (URDF support and Python bindings)
The possible extra labels are:

• build_collision (build Pinocchio with coal support)
• build_casadi (build Pinocchio with CasADi support)
• build_autodiff (build Pinocchio with CppAD support)
• build_codegen (build Pinocchio with CppADCodeGen support)
• build_extra (build Pinocchio with extra algorithms)
• build_mpfr (build Pinocchio with Boost.Multiprecision support)
• build_sdf (build Pinocchio with SDF parser)
• build_accelerate (build Pinocchio with APPLE Accelerate framework support)
• build_all (build Pinocchio with ALL the options stated above)

Thanks.
The Pinocchio development team.

[ManifoldFR]

I ended up introducing a new pinocchio::pinocchio_visualizers CMake target which is a shared library for the single base-visualizer.cpp file.

Do you think this is reasonable @jorisv ? We might rework or extend the API in the future, not sure where it would fit right now (outside of the pinocchio/utils headers which are not their own target but shared between core and parsers).

[jcarpent]

Looks good to me.

[ManifoldFR]

I forgot to update the README... I've fixed that and rebased on devel.

[ManifoldFR]

I think this is ok to merge @jcarpent :)