Make `treeSplitSize` configurable

**Is your feature request related to a problem? Please describe.**

The octree leaves can contain a certain number of particles before they try to split up into eight leaves. This is what makes the octree a _space adaptive_ container. However, the _certain number of particles_ can be chosen arbitrarily. Inside the code base, this number is called the `treeSplitThreshold`. Currently, this number is hardcoded inside the constructor of the `Octree` container. This can be seen in the following code snippet:

https://github.com/AutoPas/AutoPas/blob/79069dd645ca78d1e9215c4be7de25206f58898c/src/autopas/containers/octree/Octree.h#L65-L66

To test different configurations, it would be great if this value could be tweaked from outside the octree.

**Solution**

I would like to have an additional field (maybe inside the `ContainerSelectorInfo`) that stores the `treeSplitThreshold` and puts it inside the octree when instantiating the container.

**Additional context**

The issue refers to the pull request #610.