Analysis of compilation times, choice of template instantiation

[ManifoldFR]

Description

I am currently building some C++ code against Pinocchio. I noticed some symbols (related to calling pinocchio::urdf::buildModel<double>()) were being instantiated, which is fair enough because not all of pinocchio_parsers is explicitly template-instantiated.

After discussing with @jorisv, I went over to code-analysis-tools and followed the steps to trace the compilation time of my cpp file.
I noticed that instantiating pinocchio::urdf::buildModel<double>() took about 614ms to an overall 7100ms compilation time for my file.

For comparison, instantiating the constructor ModelTpl<double>::ModelTpl (which we think is the copy constructor) took about 900ms but if I recall correctly, this is a deliberate choice to favor runtime performance by letting the compiler inline.

The file pretty much only included pinocchio/multibody/model.hpp and pinocchio/parsers/urdf.hpp and loaded an URDF file into a pinocchio::Model.

Suggestion

The idea here would be to, at some point, profile how long it takes to instantiate non-critical functions or classes (stuff that doesn't need to get inlined) and whether to add template instantiation to these. We could do that by looking over at the examples