Started writing new load balancing routine for MPI, adding in new functions for computing load balancing quantities
Mainly cosmetic changes before fixing MPI load balancing
1. - Added new generic tree class and made KDTree inherit from it
2. - Added new Oct-tree class (currently just copied from KDTree, will be fully implemented later)
3. - Altered SphTree class in order to allow proper inheritance plus templates for code re-use
4. - Changed test files to use 'kdtree' rather than 'tree'
Cleared out unneeded routines for OctTree
Merge branch 'mpi' of https://github.com/gandalfcode/gandalf into mpi

Conflicts:
	src/KDTree.cpp
	src/SphNeighbourSearch.h
	src/SphTree.cpp