Refactor NDT map

Description

Notes for reviewer

This MR does the following changes:

• Since StaticNDTMap and DynamicNDTMap share functionality yet are quite different things, they don't conform to the same crtp interface anymore. Instead the common functionality is a base class and each class has its own API which conforms to the constraints required by their user classes.
• NDT maps had several non-template definitions done in the header files. These are moved to the source files.
• Map serialization is moved from the map publisher to the DynamicNDTMap class as a member template function. The reasoning is that, the serialization method is highly coupled with the internal representations of the dynamic ndt map and the static ndt map.
• when the serialization logic is moved, there was not much logic left in the map publisher class. So this class is removed and the remaining functionality is made into free functions.

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Checklist

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