ENH: Add IOComponent enum SCHAR for signed char (equivalent to CHAR)

The name `SCHAR` appears clearer than `CHAR`, because it is intended to specify
a _signed_ char type.

The abbreviation `SCHAR` is already used to denote `signed char` in C and C++
(`SCHAR_MAX`) as well as in HDF5 (`H5T_NATIVE_SCHAR`).

Author: N-Dekker

Modules/Core/Common/include/itkCommonEnums.h

@@ -80,7 +80,8 @@ class CommonEnums
   {
     UNKNOWNCOMPONENTTYPE,
     UCHAR,
-    CHAR,
+    SCHAR,
+    CHAR = SCHAR,
     USHORT,
     SHORT,
     UINT,