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

lib/gpu/lal_yukawa_colloid.cpp

@@ -213,7 +213,7 @@ int** YukawaColloidT::compute(const int ago, const int inum_full, const int nall
     // Make sure textures are correct if realloc by a different hybrid style
     this->resize_atom(0,nall,success);
     this->zero_timers();
-    return NULL;
+    return nullptr;
   }
 
   // load balance, returning the atom count on the device (inum)
@@ -227,7 +227,7 @@ int** YukawaColloidT::compute(const int ago, const int inum_full, const int nall
     this->build_nbor_list(inum, inum_full-inum, nall, host_x, host_type,
                           sublo, subhi, tag, nspecial, special, success);
     if (!success)
-      return NULL;
+      return nullptr;
     this->cast_rad_data(rad);
     this->hd_balancer.start_timer();
   } else {