Add a copy constructor to vtkArrayWeights.

The vtkArrayWeights header declares a destructor, therefore it
must also declare a copy constructor and assignment operator
according to the "rule of three".  In C++11 mode, clang gives
a warning when the "rule of three" is broken.

Change-Id: I0b5041100d5f00cdb0ddd709853ee49b151bba8e
Merge topic 'arrayweights-copy-ctor' into master

e6d1e307 Add a copy constructor to vtkArrayWeights.