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

src/replicate.cpp

@@ -154,7 +154,7 @@ void Replicate::command(int narg, char **arg)
   // also set atomKK for Kokkos version of Atom class
 
   Atom *old = atom;
-  atomKK = NULL;
+  atomKK = nullptr;
   if (lmp->kokkos) atom = atomKK = new AtomKokkos(lmp);
   else atom = new Atom(lmp);