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/CauchyBorn.cpp

@@ -426,7 +426,7 @@ namespace ATC {
       for (INDEX j=i; j<3; j++)
         s(i,j) += factor * dd(i,j);
 
-    // If f_W is not NULL then append thermal contribution.
+    // If f_W is not nullptr then append thermal contribution.
     if (F_w) *F_w += 0.5*kb*T*log(detD);
   }
   //============================================================================