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/dump_atom.cpp

@@ -34,7 +34,7 @@ DumpAtom::DumpAtom(LAMMPS *lmp, int narg, char **arg) : Dump(lmp, narg, arg)
   image_flag = 0;
   buffer_allow = 1;
   buffer_flag = 1;
-  format_default = NULL;
+  format_default = nullptr;
 }
 
 /* ---------------------------------------------------------------------- */