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/GPU/pair_colloid_gpu.cpp

@@ -158,7 +158,7 @@ void PairColloidGPU::init_style()
   }
   double cell_size = sqrt(maxcut) + neighbor->skin;
 
-  int **_form = NULL;
+  int **_form = nullptr;
   int n=atom->ntypes;
   memory->create(_form,n+1,n+1,"colloid/gpu:_form");
   for (int i = 1; i <= n; i++) {