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/npair.cpp

@@ -27,10 +27,10 @@ using namespace LAMMPS_NS;
 /* ---------------------------------------------------------------------- */
 
 NPair::NPair(LAMMPS *lmp)
-  : Pointers(lmp), nb(NULL), ns(NULL), bins(NULL), stencil(NULL)
+  : Pointers(lmp), nb(nullptr), ns(nullptr), bins(nullptr), stencil(nullptr)
 {
   last_build = -1;
-  mycutneighsq = NULL;
+  mycutneighsq = nullptr;
   molecular = atom->molecular;
   copymode = 0;
   execution_space = Host;