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/FieldManager.cpp

@@ -271,7 +271,7 @@ typedef PerAtomQuantity<double> PAQ;
       u = new AtfShapeFunctionMdProjection(atc_,restricted,VELOCITY);
     }
     else {
-      DENS_MAN * q = NULL;
+      DENS_MAN * q = nullptr;
       if (atc_->kernel_on_the_fly()) {
         if (atc_->kernel_based()) {
           q = new OnTheFlyKernelAccumulationNormalized(atc_, atomic,