New SmartPointer issues

Following the recent update to ITK’s SmartPointers, I had one issue with compilation of SimpleITK:

SimpleITK/Code/BasicFilters/src/sitkCreateInterpolator.hxx:143:14: error: conversion from 'long int' to 'itk::InterpolateImageFunction, double>::Pointer {aka itk::SmartPointer, double> >}' is ambiguous

Which boils down to:

 itk::SmartPointer<ITKType> SomeFunction() 
return NULL;

This was an easy fix. I just globally replaced NULL with the SITK_NULLPTR macro which is defined as nullptr c++11.