This fix is in reference to bug report 0014340, and is an extension of the
change made in commit 84b126f9. vtkAbstractArray and its children have the
convention of a fast, unsafe method for setting elements (SetXXX) and a safer,
slower method (InsertXXX). In the prior commit, this convention was extended to
setting with variant values. In vtkAbstractArray, InsertVariantValue was not a
pure virtual method, and it called the unsafe SetXXX method (which led to
problems, see the bug report). By making this method pure virtual, we ensure
that InsertXXX is indeed safe for each container. It also enforces the pattern
in place for SetXXX vs InsertXXX with other variable types.