blockMesh: added experimental fast-merge algorithm

The standard merge-algorithm is N^2 over the face-points and uses a
geometric proximity test for the merge.  These are both choices for
implementation simplicity and are rather inefficient for large meshes.
I have now implemented an experimental linear topological merge
algorithm which is VERY fast and effective for meshes of any size.
Currently it will merge internal faces on meshes of arbitrary complexity
but does not yet handle edge or face collapse needed for wedges and
other degenerate blocks.

The new fast-merge algorithm may be selected using the optional
"fastMerge" entry:

fastMerge yes;

and if not present the standard N^2 algorithm will be used.

Henry G. Weller
CFD Direct