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/REPLICA/fix_event.cpp

@@ -27,8 +27,8 @@ using namespace FixConst;
 /* ---------------------------------------------------------------------- */
 
 FixEvent::FixEvent(LAMMPS *lmp, int narg, char **arg) :
-  Fix(lmp, narg, arg), xevent(NULL), xold(NULL), vold(NULL),
-  imageold(NULL), xorig(NULL), vorig(NULL), imageorig(NULL)
+  Fix(lmp, narg, arg), xevent(nullptr), xold(nullptr), vold(nullptr),
+  imageold(nullptr), xorig(nullptr), vorig(nullptr), imageorig(nullptr)
 {
   if (narg != 3) error->all(FLERR,"Illegal fix event command");