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_halt.cpp

@@ -37,7 +37,7 @@ enum{NOMSG,YESMSG};
 /* ---------------------------------------------------------------------- */
 
 FixHalt::FixHalt(LAMMPS *lmp, int narg, char **arg) :
-  Fix(lmp, narg, arg), idvar(NULL), dlimit_path(NULL)
+  Fix(lmp, narg, arg), idvar(nullptr), dlimit_path(nullptr)
 {
   if (narg < 7) error->all(FLERR,"Illegal fix halt command");
   nevery = utils::inumeric(FLERR,arg[3],false,lmp);
@@ -45,7 +45,7 @@ FixHalt::FixHalt(LAMMPS *lmp, int narg, char **arg) :
 
   // comparison args
 
-  idvar = NULL;
+  idvar = nullptr;
   int iarg = 4;
 
   if (strcmp(arg[iarg],"tlimit") == 0) {