Initial work on use of k-D tree index for ungridded colocation.
Further work in progress for kernel optimisation.
Merge remote-tracking branch 'origin/devel' into kerneloptimisation

Conflicts:
	jasmin_cis/col_framework.py
	jasmin_cis/col_implementations.py
More work on k-D tree with haversine distances.
Added spherical trigonometric calculation of nearest and furthest points for k-D tree separation colocation constraint.
Minor changes to code for testing.
Renamed IndexedSepConstraint to SepConstraintKdtree.
Updated docstring style to RST.
Merge remote-tracking branch 'origin/devel' into kerneloptimisation