New header convention

[jorisv]

This PR is a POC about include convention.

Right now, there is include convention. This lead to some issues:

• Code discrepancy
• Cyclic include
• LSP is not working well (since some includes are missing)
• Big dependency chain (since they are probable unused include in most file)

Since Pinocchio is header only, we can't use the include what you use convention since it will create a lot of cycle.

In this PR we propose to change the include convention as the following:

• Public headers: xxx.hpp
• Private headers:
  • Definition: xxx-def.hxx
  • Declaration: xxx-decl.hxx
  • Explicit template instantiation: xxx-tpl.hxx

With the following rules:

• Private header can't include anything
• Public header can include private header

This should prevent cyclic include and with some .clangd trick will improve the LSP processing.

All is described in development/convention.md.

Constraints

• Public API should be preserved
• Authors should be preserved

[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.