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/compute_temp_region.cpp

@@ -29,7 +29,7 @@ using namespace LAMMPS_NS;
 
 ComputeTempRegion::ComputeTempRegion(LAMMPS *lmp, int narg, char **arg) :
   Compute(lmp, narg, arg),
-  idregion(NULL)
+  idregion(nullptr)
 {
   if (narg != 4) error->all(FLERR,"Illegal compute temp/region command");
 
@@ -48,7 +48,7 @@ ComputeTempRegion::ComputeTempRegion(LAMMPS *lmp, int narg, char **arg) :
   tempbias = 1;
 
   maxbias = 0;
-  vbiasall = NULL;
+  vbiasall = nullptr;
   vector = new double[size_vector];
 }