Remove Barnes-Hut and MMM1D GPU, add LB multi-GPU support

[jngrad]

We are reducing the complexity of the ESPResSo CUDA code to introduce multi-GPU support. This effort will require replacing P3M GPU by an upcoming ScaFaCoS GPU library, offloading LB GPU to waLBerla, removing Barnes-Hut and removing MMM1D GPU.

Description of changes:

• add experimental multi-GPU support for LB
• API changes:
  • remove Barnes-Hut feature
  • remove MMM1D GPU