Partitioner refactor 1 (#49)

* Introduce general get_neighbours

* Move to get_neighbours

* Switch to index system for neighbours

* num_neighbours vector

* dims vector

* Offsets vector

* id vector

* Halos vector

* Separate ids_tblr

* Get all neighbours at once

* Loop for global dims and offsets

* Dimid vector

* num_vid vector

* ids_vid and halos_vid vectors

* dim_names and dim_letters vectors

* _global and _global_new vectors

* _local_ext and _local_ext_new vectors

* _global_ext vector

* _num_procs vector

* vector of vector for bboxes

* vector of vector of vectors for bbox

* Looping in ZoltanPartitioner

* Introduce NNBRS for generality

* dim_names; avoid unnecessary vectors

* Use NDIMS and NNBRS

* Reorder discover_neighbours

* Collapse two dims of bbox arrays

* Combine loops in discover_neighbours

* Set up neighbours with NNBRS

* Declare labels in class header

* Combine setup loops

* Revert changes to discover_neighbours

* Combine loops