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/atc/SpeciesTimeIntegrator.h

@@ -71,7 +71,7 @@ namespace ATC {
       const std::map<std::string,std::pair<MolSize,int> > & moleculeIds);
         
     // destructor
-    virtual ~SpeciesIntegrationMethod() {nodalAtomicMassDensity_=NULL;};
+    virtual ~SpeciesIntegrationMethod() {nodalAtomicMassDensity_=nullptr;};
 
     /** create and get necessary transfer operators */
     virtual void construct_transfers();