Only test circumspheres that overlap the current processor boundary.

Sphere overlap test is just a findNearest test, no need for findSphere.

Function for cell: internalOrBoundaryDualVertex.

Insertion or removal can only happen between real, internal points, not between
real internal and referred internal points.

Function (referredInternalOrBoundary) name change, more accurate.

Rename hierarchicalDensityWeightedStochastic to autoDensity.

Remove densityWeightedStochastic, superceded and difficult to parallelise.

UniformGrid parallelised.

Reinstating writing out cell size field.