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/PoissonSolver.cpp

@@ -33,9 +33,9 @@ PoissonSolver::PoissonSolver(
     fieldName_(fieldName),
     rhsMask_(rhsMask),
     linear_(false),
-    solver_(NULL),
-    solverNL_(NULL),
-    tangent_(NULL),
+    solver_(nullptr),
+    solverNL_(nullptr),
+    tangent_(nullptr),
     solverType_(solverType),
     solverTol_(0),
     solverMaxIter_(0),