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/fix_wall.cpp

@@ -51,7 +51,7 @@ FixWall::FixWall(LAMMPS *lmp, int narg, char **arg) :
   fldflag = 0;
   int pbcflag = 0;
 
-  for (int i = 0; i < 6; i++) xstr[i] = estr[i] = sstr[i] = NULL;
+  for (int i = 0; i < 6; i++) xstr[i] = estr[i] = sstr[i] = nullptr;
 
   int iarg = 3;
   while (iarg < narg) {