ENH: Added "signed char" as a distinct scalar type.  For historical reasons, the C++ language treats signed char as a distinct type from char.  Plain-old char might not even be signed.  This change will guarantee we have a signed 8-bit integer type available.  Also added GetDataTypeMin/GetDataTypeMax/GetDataTypeRange static overloads to vtkDataArray that accept the type id.  The non-static versions now call the static versions with the type of data in the array.