Shorten header structure and merge Model and Sensors libraries

[traversaro]

Fix #132 . #132 was a prerequisite, but in following PRs the merge of Model and Sensors will permit us to add a SensorsList to the iDynTree::Model. cc @Nicogene

Sorry for the gigantic PR, but that is unfortunately necessary given the nature of the change, however the single commit are smaller.

[traversaro]

Python CI is failing with:

2023-09-06T08:22:17.5615831Z   -- Installing: /tmp/pip-req-build-9quuo_de/build/lib.linux-x86_64-cpython-310/idyntree/include/iDynTree/KalmanFilter.h
2023-09-06T08:22:17.5618413Z   CMake Error at build/temp.linux-x86_64-cpython-310_BuildAndInstall/src/estimation/cmake_install.cmake:69 (file):
2023-09-06T08:22:17.5620628Z     file INSTALL cannot find
2023-09-06T08:22:17.5623384Z     "/tmp/pip-req-build-9quuo_de/src/estimation/include/iDynTree/estimation":
2023-09-06T08:22:17.5625698Z     No such file or directory.
2023-09-06T08:22:17.5628154Z   Call Stack (most recent call first):
2023-09-06T08:22:17.5630850Z     build/temp.linux-x86_64-cpython-310_BuildAndInstall/src/cmake_install.cmake:62 (include)
2023-09-06T08:22:17.5633375Z     build/temp.linux-x86_64-cpython-310_BuildAndInstall/cmake_install.cmake:52 (include)

[Nicogene]

🚀

[traversaro]

Failures are unrelated, merging.