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/angle_zero.cpp

@@ -128,7 +128,7 @@ void AngleZero::read_restart(FILE *fp)
   allocate();
 
   if (comm->me == 0) {
-    utils::sfread(FLERR,&theta0[1],sizeof(double),atom->nangletypes,fp,NULL,error);
+    utils::sfread(FLERR,&theta0[1],sizeof(double),atom->nangletypes,fp,nullptr,error);
   }
   MPI_Bcast(&theta0[1],atom->nangletypes,MPI_DOUBLE,0,world);