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/nbin.cpp

@@ -27,9 +27,9 @@ NBin::NBin(LAMMPS *lmp) : Pointers(lmp)
 {
   last_bin = -1;
   mbins = maxbin = maxatom = 0;
-  binhead = NULL;
-  bins = NULL;
-  atom2bin = NULL;
+  binhead = nullptr;
+  bins = nullptr;
+  atom2bin = nullptr;
 
   neighbor->last_setup_bins = -1;