Convert np.int8 to VTK_SIGNED_CHAR, not VTK_CHAR

David Gobbirequested to merge
dgobbi/vtk:numpy-int8-signed into master

Conversion of numpy's int8 datatype to VTK_CHAR was incorrect because int8 is always signed, but the signedness of VTK_CHAR is platform-dependent. The correct conversion is to VTK_SIGNED_CHAR. Also add a test for the conversion.

Backport: release

Edited by David Gobbi

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