particle_groups: add from_region

Constructs a particle group from a region. The selected particles can
either be in the region or outside of it. Both will be calculated at the
same time from the region module.

Warning: Host variant crashes due to missing resize() of raw_array.
fix two_particles test
utility/raw_array: add reserve(), resize(), capacity()

Add very simple size-changing functions to raw_array.
Note there is no guarantee that the data stored in the array is
preserved after a calling reserve() or resize().
particle_group: add function to copy particle group into particle

This function will copy only position, image, velocity, mass, species.
Others do not make much sense in a new particle instance (e.g. id, force
or auxiliary variables).
from_region: make conversation to particle instance accessible