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/USER-MISC/pair_tersoff_table.cpp

@@ -755,7 +755,7 @@ void PairTersoffTable::coeff(int narg, char **arg)
     error->all(FLERR,"Incorrect args for pair coefficients");
 
   // read args that map atom types to elements in potential file
-  // map[i] = which element the Ith atom type is, -1 if NULL
+  // map[i] = which element the Ith atom type is, -1 if nullptr
   // nelements = # of unique elements
   // elements = list of element names
 
@@ -764,7 +764,7 @@ void PairTersoffTable::coeff(int narg, char **arg)
     delete [] elements;
   }
   elements = new char*[atom->ntypes];
-  for (i = 0; i < atom->ntypes; i++) elements[i] = NULL;
+  for (i = 0; i < atom->ntypes; i++) elements[i] = nullptr;
 
   nelements = 0;
   for (i = 3; i < narg; i++) {