implemented NewFastKdTreePair()

now the FastKdTreeMemoryManager allocates two linear FastKdTree memory
spaces for a left and right FastKdTree pointer.
corrected a comment for the fastkdtreememoryallocator.h
NewFastKdTreePair
changed a few methods for FastKdTree

it's only possible to get, but not set the right child tree.
removed not useful todo