Add missing IsPODPointer check, for overload resolution.

The vtkWrapPython_ArgCheckString() method was not adding the necessary
arg check characters for PODPointer-type parameters.  This can potentially
result in incorrect method overload resolution.

Change-Id: I15dd969fa4997afb23708395bcc21dc58298a3fb
Merge topic 'python-pointer-cleanup' into master

9ffbf1dc Add missing IsPODPointer check, for overload resolution.