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/ParSparseMatrix.h

@@ -135,9 +135,9 @@ namespace ATC_matrix {
     Avar.hasTemplate_ = Ap.hasTemplate_;
 
     // Avoid catastrophe
-    Ap._val = NULL;
-    Ap._ja  = NULL;
-    Ap._ia  = NULL;
+    Ap._val = nullptr;
+    Ap._ja  = nullptr;
+    Ap._ia  = nullptr;
   }