Nbody example

[psychocoderHPC]

First of all thank you for writing the Nbody example.
I have a few small wishes:

• can you split the code in logical separate files and maybe folder
• please add some documentation what NBODY_USE_SHARED_TREE, NBODY_USE_SHARED mean and what is the conceptional differences if I select it
• add basic documentation to classes and methods

[bertwesarg]

The N-Body example is an extension to the N-Body example from the llama repo https://github.com/ComputationalRadiationPhysics/llama/tree/master/examples/nbody. Thus, please direct your small wishes there first, as I think all of them originate from there.