T2345: Replace instances of NULL with nullptr

The following changes have been applied to src and lib folders:
regex replace: ([^"_])NULL ⇒ \1nullptr (8968 chgs in src, 1153 in lib)
Manually find/change: (void \*) nullptr ⇒ nullptr (1 case)
regex find: ".*?nullptr.*?"
  Manually ~14 cases back to "NULL" in src, ~2 in lib
  regex finds a few false positive where nullptr appears between two
  strings in a function call

lib/atc/SparseVector.h

@@ -88,7 +88,7 @@ protected:
   //@{
   SparseVector(const Matrix<T> &c);
   SparseVector<T>& operator=(Matrix<T> &c);
-  T* ptr() const {return NULL; } 
+  T* ptr() const {return nullptr; } 
   //@}
   
   STORE data_;  //*> sparse data structure