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/neigh_request.cpp

@@ -71,12 +71,12 @@ NeighRequest::NeighRequest(LAMMPS *lmp) : Pointers(lmp)
   // skip info, default is no skipping
 
   skip = 0;
-  iskip = NULL;
-  ijskip = NULL;
+  iskip = nullptr;
+  ijskip = nullptr;
 
   // only set when command = 1;
 
-  command_style = NULL;
+  command_style = nullptr;
 
   // info set by Neighbor class when morphing original requests
 
@@ -217,8 +217,8 @@ void NeighRequest::copy_request(NeighRequest *other, int skipflag)
   cut = other->cut;
   cutoff = other->cutoff;
 
-  iskip = NULL;
-  ijskip = NULL;
+  iskip = nullptr;
+  ijskip = nullptr;
 
   if (!skipflag) return;