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.cpp

@@ -37,10 +37,10 @@ int Compute::instance_total = 0;
 
 Compute::Compute(LAMMPS *lmp, int narg, char **arg) :
   Pointers(lmp),
-  id(NULL), style(NULL),
-  vector(NULL), array(NULL), vector_atom(NULL),
-  array_atom(NULL), vector_local(NULL), array_local(NULL), extlist(NULL),
-  tlist(NULL), vbiasall(NULL)
+  id(nullptr), style(nullptr),
+  vector(nullptr), array(nullptr), vector_atom(nullptr),
+  array_atom(nullptr), vector_local(nullptr), array_local(nullptr), extlist(nullptr),
+  tlist(nullptr), vbiasall(nullptr)
 {
   instance_me = instance_total++;