Add namespaces

Introduce namespaces to SLAM lib. Using namespaces ensures better code organization and readability, but also prevent polluting outer namespace, names conflicts, or hiding symbols.

• All SLAM lib now lies in LidarSlam namespace.
• All helpers defined in LidarSlam/Utilities.h are moved to Utils namespace. Processing duration helpers are moved in nested namespace Timer.
• All helpers locally defined in a single cxx file are moved to an anonymous namespace inside Utils, so that they are not exported nor available from outside.

To ensure using the correct files, the include guards in SLAM lib headers are replaced by #pragma once declarations.

WARNING : all external code directly using the LidarSlam lib won't compile anymore until you add the namespace.

@nick.laurenson @julia.sanchez @lea.vauchier PTAL