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

lib/gpu/lal_neighbor.cpp

@@ -158,7 +158,7 @@ void Neighbor::alloc(bool &success) {
     }
     _c_bytes+=nbor_host.device.row_bytes()+dev_numj_host.row_bytes();
   } else {
-    // Some OpenCL implementations return errors for NULL pointers as args
+    // Some OpenCL implementations return errors for nullptr pointers as args
     nbor_host.device.view(dev_nbor);
     dev_numj_host.view(dev_nbor);
   }