This adds automatic deletion of cells inside small gaps. This is
generally used to avoid having excessive numbers of cells in irrelevant
areas of a geometry. It is nearly the opposite of automatic gap refinement
 - that refines cells to resolve the gap; this functionality removes cells
to not mesh the gap.

The proximity handling will remove those cells which are inside 'thin' gaps
where 'thin' is defined as a distance of 2*'blockLevel'
It will
- detect surfaces which have the new 'blockLevel' specification
- convert this to a minimum gap distance
- detect cells which are inside this gap
- remove these cells and add exposed faces to the nearest 'real' patch