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/create_atoms.h

@@ -62,7 +62,7 @@ class CreateAtoms : protected Pointers {
   void add_random();
   void add_lattice();
   void loop_lattice(int);
-  void add_molecule(double *, double * = NULL);
+  void add_molecule(double *, double * = nullptr);
   int vartest(double *);      // evaluate a variable with new atom position
 };