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/SpeciesTimeIntegrator.cpp

@@ -108,9 +108,9 @@ namespace ATC {
     timeFilter_(speciesTimeIntegrator->time_filter()),
     massDensity_(atc_->field(MASS_DENSITY)),
     nodalAtomicMassDensityOut_(atc_->nodal_atomic_field(MASS_DENSITY)),
-    nodalAtomicMassDensity_(NULL),
+    nodalAtomicMassDensity_(nullptr),
     speciesConcentration_(atc_->field(SPECIES_CONCENTRATION)),
-    nodalAtomicSpeciesConcentration_(NULL),
+    nodalAtomicSpeciesConcentration_(nullptr),
     nodalAtomicSpeciesConcentrationFiltered_(speciesTimeIntegrator->nodal_atomic_species_concentration_filtered()),
     moleculeIds_(moleculeIds)
   {