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/fix_numdiff.cpp

@@ -40,8 +40,8 @@ using namespace FixConst;
 /* ---------------------------------------------------------------------- */
 
 FixNumDiff::FixNumDiff(LAMMPS *lmp, int narg, char **arg) :
-  Fix(lmp, narg, arg), id_pe(NULL), numdiff_forces(NULL),
-  temp_x(NULL), temp_f(NULL)
+  Fix(lmp, narg, arg), id_pe(nullptr), numdiff_forces(nullptr),
+  temp_x(nullptr), temp_f(nullptr)
 {
   if (narg < 5) error->all(FLERR,"Illegal fix numdiff command");