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/ExtrinsicModelTwoTemperature.cpp

@@ -26,7 +26,7 @@ namespace ATC {
                                  string matFileName) :
     ExtrinsicModel(modelManager,modelType,matFileName),
     electronTimeIntegration_(TimeIntegrator::IMPLICIT),
-    temperatureIntegrator_(NULL),
+    temperatureIntegrator_(nullptr),
     nsubcycle_(1),
     exchangeFlag_(true), 
     baseSize_(0)
@@ -164,7 +164,7 @@ namespace ATC {
       rhsMask(ELECTRON_TEMPERATURE,i) = atc_->fieldMask_(ELECTRON_TEMPERATURE,i);
     }
     if (electronTimeIntegration_ == TimeIntegrator::NONE) {
-      temperatureIntegrator_ = NULL;
+      temperatureIntegrator_ = nullptr;
       return;
     }
     if (temperatureIntegrator_) delete temperatureIntegrator_;