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/respa.cpp

@@ -46,8 +46,8 @@ using namespace LAMMPS_NS;
 
 Respa::Respa(LAMMPS *lmp, int narg, char **arg) :
   Integrate(lmp, narg, arg),
-  step(NULL), loop(NULL), hybrid_level(NULL), hybrid_compute(NULL),
-  newton(NULL), fix_respa(NULL)
+  step(nullptr), loop(nullptr), hybrid_level(nullptr), hybrid_compute(nullptr),
+  newton(nullptr), fix_respa(nullptr)
 {
   nhybrid_styles = 0;
   if (narg < 1) error->all(FLERR,"Illegal run_style respa command");