Deprecate the obsolete vtkFastNumericConversion class.

This class was designed as a work-around for some serious performance
problems with the i387 floating-point unit in the x86 family of CPUs.
It is largely irrelevant for modern x86 CPUs which typically use sse
for floating-point computations.  Even though it might give a small
performance boost, it gives an imprecise result.

Change-Id: I65109d827c38bd6dd0ea6ace191885685b6674e3
Merge topic 'remove-fast-numeric-conversion' into master

3bbd7d2e Deprecate the obsolete vtkFastNumericConversion class.