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/FE_Engine.cpp

@@ -32,7 +32,7 @@ namespace ATC{
   //-----------------------------------------------------------------
   FE_Engine::FE_Engine(MPI_Comm comm)
     : communicator_(comm),
-      feMesh_(NULL),
+      feMesh_(nullptr),
       initialized_(false),
       outputManager_()
   {