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_pe_atom.cpp

@@ -33,7 +33,7 @@ using namespace LAMMPS_NS;
 
 ComputePEAtom::ComputePEAtom(LAMMPS *lmp, int narg, char **arg) :
   Compute(lmp, narg, arg),
-  energy(NULL)
+  energy(nullptr)
 {
   if (narg < 3) error->all(FLERR,"Illegal compute pe/atom command");