Remove special characters from VTK source files.

The source was searched with "grep -l [^ -~]" to find characters outside
of the printable ASCII range 0x20 to 0x7e.  The worst offender is the
en-dash character, which is visually indistinguishable from the minus
character in most code editors.  Several tabs were also found, which
is surprising since they should be blocked by the commit hooks.