Should be reintroduced for newer version of NumPy

* include registration: add failing unit test

* include registration: break circular include loop

otherwise, when a user include directly this header without fwd.hpp, as
in the include.cpp regression test:

Dans le fichier inclus depuis ../include/eigenpy/fwd.hpp:35,
                 depuis ../include/eigenpy/registration.hpp:9,
                 depuis ../unittest/include.cpp:7:
../include/eigenpy/expose.hpp: Dans la fonction « void eigenpy::expose() »:
../include/eigenpy/expose.hpp:27:9: erreur: « register_symbolic_link_to_registered_type » n'a pas été déclaré dans cette portée
   27 |     if(!register_symbolic_link_to_registered_type<T>())
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../include/eigenpy/expose.hpp:27:52: erreur: expected primary-expression before « > » token
   27 |     if(!register_symbolic_link_to_registered_type<T>())
      |                                                    ^
../include/eigenpy/expose.hpp:27:50: attention: le non logique est toujours appliqué au membre de gauche de la comparaison [-Wlogical-not-parentheses]
   27 |     if(!register_symbolic_link_to_registered_type<T>())
      |                                                  ^
../include/eigenpy/expose.hpp:27:54: erreur: expected primary-expression before « ) » token
   27 |     if(!register_symbolic_link_to_registered_type<T>())
      |                                                      ^

fix:

CMake Warning (dev) at unittest/CMakeLists.txt:10 (ADD_LIBRARY):
  Policy CMP0115 is not set: Source file extensions must be explicit.  Run
  "cmake --help-policy CMP0115" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  File:

    …/eigenpy/unittest/eigen_ref.cpp
Call Stack (most recent call first):
  unittest/CMakeLists.txt:33 (ADD_LIB_UNIT_TEST)
This warning is for project developers.  Use -Wno-dev to suppress it.

ref. https://github.com/stack-of-tasks/tsid/pull/129

No more useful