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_temp_rescale.cpp

@@ -37,7 +37,7 @@ enum{CONSTANT,EQUAL};
 
 FixTempRescale::FixTempRescale(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
-  tstr(NULL), id_temp(NULL), tflag(0)
+  tstr(nullptr), id_temp(nullptr), tflag(0)
 {
   if (narg < 8) error->all(FLERR,"Illegal fix temp/rescale command");
 
@@ -50,7 +50,7 @@ FixTempRescale::FixTempRescale(LAMMPS *lmp, int narg, char **arg) :
   extscalar = 1;
   dynamic_group_allow = 1;
 
-  tstr = NULL;
+  tstr = nullptr;
   if (strstr(arg[4],"v_") == arg[4]) {
     int n = strlen(&arg[4][2]) + 1;
     tstr = new char[n];
@@ -278,5 +278,5 @@ void *FixTempRescale::extract(const char *str, int &dim)
     dim = 0;
     return &t_target;
   }
-  return NULL;
+  return nullptr;
 }