Updating type usage [pepc-v] and fixing possible memory corruption for `grow_and_traverse`

More stringent typing has been introduced a couple of years back. Attempting to update pepc-v in that regard (not claiming it's complete). Said frontend is also updated for a safer use of the particle array's length with re-allocations after the particle number changed.

A general memory corruption can occur in pepc_grow_and_traverse_for_others when not carefully used. Two fixes are introduced to ensure it does not happen again.