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_pppm.cpp

@@ -35,7 +35,7 @@ PPPMT::PPPM() : _allocated(false), _compiled(false),
                                   _max_bytes(0) {
   device=&global_device;
   ans=new Answer<numtyp,acctyp>();
-  pppm_program=NULL;
+  pppm_program=nullptr;
 }
 
 template <class numtyp, class acctyp, class grdtyp, class grdtyp4>