Homogenize internal namespace

[jorisv]

Pinocchio use different internal namespace for non public API. This should be homogenize to improve code readability.

Internal namespace are the following:

• details
• internal
• impl
• helper
• optimized
• fix

There is not a clear convention for the use of public namespace.
Parsers are set in a specific namespace.
Mathematical functions overload are in math namespace.
boost serialization functions are in serialization namespace.
It's less clear why a namespace exists for other functions and it's not releated to an include directory.

• buildModels
• cholesky
• container
• forceSet
• motionSet
• math
• mjcf
• urdf
• sdf
• srdf
• quaternion
• rpy
• serialization