Fix vtkDataReader with signed char

The vtkDataWriter class uses the "signed_char" identifier for signed
char data but the vtkDataReader class only checks for "char". Now it
accepts either "char" or "signed_char".

This fixes VTK bug #13744.

Change-Id: I7f0c7614e4d01a0b700ab84a370d1896c63b18d1
Merge topic '13744-data-reader-signed-char' into master

d92bf40e Fix vtkDataReader with signed char