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/ExtrinsicModelElectrostatic.cpp

@@ -38,15 +38,15 @@ namespace ATC {
     poissonSolverType_(DIRECT), // ITERATIVE | DIRECT
     poissonSolverTol_(0),
     poissonSolverMaxIter_(0),
-    poissonSolver_(NULL),
+    poissonSolver_(nullptr),
     maxSolves_(0),
     baseSize_(0),
-    chargeRegulator_(NULL),
+    chargeRegulator_(nullptr),
     useSlab_(false),
     includeShortRange_(true),
-    atomForces_(NULL),
-    nodalAtomicCharge_(NULL),
-    nodalAtomicGhostCharge_(NULL)
+    atomForces_(nullptr),
+    nodalAtomicCharge_(nullptr),
+    nodalAtomicGhostCharge_(nullptr)
   {
      physicsModel_ = new PhysicsModelSpeciesElectrostatic(matFileName);
      // set up correct masks for coupling