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/irregular.h

@@ -30,7 +30,7 @@ class Irregular : protected Pointers {
   Irregular(class LAMMPS *);
   ~Irregular();
   void migrate_atoms(int sortflag = 0, int preassign = 0,
-                     int *procassign = NULL);
+                     int *procassign = nullptr);
   int migrate_check();
   int create_data(int, int *, int sortflag = 0);
   int create_data_grouped(int, int *, int sortflag = 0);