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

src/update.cpp

@@ -48,13 +48,13 @@ Update::Update(LAMMPS *lmp) : Pointers(lmp)
 
   eflag_global = vflag_global = -1;
 
-  unit_style = NULL;
+  unit_style = nullptr;
   set_units("lj");
 
-  integrate_style = NULL;
-  integrate = NULL;
-  minimize_style = NULL;
-  minimize = NULL;
+  integrate_style = nullptr;
+  integrate = nullptr;
+  minimize_style = nullptr;
+  minimize = nullptr;
 
   integrate_map = new IntegrateCreatorMap();