Generalize PicManager for multiple particle containers

[rammann]

This PR generalizes the PicManager class to allow for multiple particle containers.

Changes:

• We now additionally store a vector of shared_ptrs to the particle containers.

std::vector<std::shared_ptr<pc>> pcontainers_m;

• Functions to get, set and add new particle containers to the PicManager

std::shared_ptr<pc> getParticleContainer(size_t i)
std::shared_ptr<pc> getParticleContainer(size_t i)
size_t addParticleContainer(std::shared_ptr<pc> pcontainer)

Backwards compatibility

TODO

How does this affect load balancing

TODO