Multi-dimensional nodeset performance regression in v1.8

[degremont]

Multi dimensional nodeset is slower with CS v1.8 compared to CS 1.7.3. Between 25 and 30% slower depending on the operation.

 TESTNAME                 v1.7.3       v1.8
 NodeSet2DContains       0.29597    0.35862       21%
 NodeSet2DIter           0.08717    0.11292       30%
 NodeSet2DParse          0.22292    0.32376       45%
 NodeSet2DRemove         0.29858    0.35797       20%
 NodeSet2DStr            0.09125    0.11323       24%
 NodeSet2DUnion          0.10307    0.13719       33%
 NodeSet2DcPickle        0.04829    0.04754       -2%

I bisect'd this to this commit 95b3c435c9fef1f2678a23a59aea86f8459fde85

RangeSet: remove _wrap_set_op() for Python 3 support (#239)

This patch uses a technique already used in RangeSetND to subclass the
built-in set in RangeSet. The thing is to avoid calling any set methods
that return new sets, but use the mutable methods instead.

Tested with Python 2.6.6, 2.7.5 and 3.4.5.

Change-Id: I53a4f044c7ffc6ac4c6f1b54eaa57ffc901b0898

Don't know what to do with that for now...