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/PerAtomQuantity-inl.h

@@ -24,8 +24,8 @@ namespace ATC {
     atomType_(atomType),
     nCols_(nCols),
     quantityToLammps_(atc_.atc_to_lammps_map()),
-    lammpsScalar_(NULL),
-    lammpsVector_(NULL)
+    lammpsScalar_(nullptr),
+    lammpsVector_(nullptr)
   {
     // do nothing
   }
@@ -452,7 +452,7 @@ namespace ATC {
     lammpsInterface_(LammpsInterface::instance()),
     atomType_(atomType),
     quantityToLammps_(atc_.atc_to_lammps_map()),
-    lammpsScalar_(NULL)
+    lammpsScalar_(nullptr)
   {
     // do nothing
   }
@@ -610,8 +610,8 @@ namespace ATC {
     nCols_(nCols),
     maxEntriesPerRow_(maxEntriesPerRow),
     quantityToLammps_(atc_.atc_to_lammps_map()),
-    lammpsVector_(NULL),
-    lammpsColIndices_(NULL)
+    lammpsVector_(nullptr),
+    lammpsColIndices_(nullptr)
   {
     // do nothing
   }