Merge C++/Python context

[jorisv]

Pinocchio use context file to be build against specific scalar type. There are specific context file for python and C++ build. Both contains some overlapping code and should be merged.
This will allow to avoid code duplication, but also to use explicit template instantiation C++ library to build python bindings.

Context file are used:

• To construct a python binding for a specific scalar type.
• To construct explicitly template instantiate C++ Pinocchio function for specific scalar type (pinocchio_default, pinocchio_casadi, pinocchio_cppad, pinocchio_cppadcg).

Both context file all contains some redundant information:

• Scalar to use
• Function specialization for custom scalar
• Typedef of main types with custom scalar
• Include to manage custom scalar
• Define to skip some algorithm not supported by a custom scalar type

The only part that should be keep splitted is the python specific part (eigenpy and pinocchio::python specialization).