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/atc/ExtrinsicModel.cpp

@@ -114,7 +114,7 @@ namespace ATC {
       ATC::LammpsInterface::instance()->print_msg_once(ss.str());
       myModel = new ExtrinsicModelElectrostatic
         (this,modelType,matFileName);
-    } else myModel = NULL;
+    } else myModel = nullptr;
     extrinsicModels_.push_back(myModel);
 
      // add new fields to fields data
@@ -157,7 +157,7 @@ namespace ATC {
       for(imodel=extrinsicModels_.begin(); imodel!=extrinsicModels_.end(); imodel++) {
         if ((*imodel)->model_type()==type) return *imodel;
       }
-      return NULL;
+      return nullptr;
     }
 
 
@@ -343,7 +343,7 @@ namespace ATC {
     atc_(modelManager->atc()),
     modelManager_(modelManager),
     modelType_(modelType),
-    physicsModel_(NULL)
+    physicsModel_(nullptr)
   {
     rhsMaskIntrinsic_.reset(NUM_FIELDS,NUM_FLUX);
     rhsMaskIntrinsic_ = false;