Composition over Inheritance

We have a few classes (Tracker, TrackPerson, TrackPoint) which inherit, where a data member would suffice. They don't expand on the base-class, but instead use them as data containers. The is-A relationship isn't there.

C++ Core Guideline with a bit more reasoning: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c120-use-class-hierarchies-to-represent-concepts-with-inherent-hierarchical-structure-only

So these base classes should be moved to member variables.

Currently blocked by #349