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/SchrodingerSolver.cpp

@@ -646,7 +646,7 @@ double fermi_dirac(const double E, const double T)
     double mu, double D
   ) :
   SliceSchrodingerPoissonSolver(atc,schrodingerSolver,poissonSolver,physicsModel,maxConsistencyIter,maxConstraintIter,oneDconserve,0,0),
-  solver_(NULL), 
+  solver_(nullptr), 
   mobility_(mu),diffusivity_(D)
   {
     Ef0_ = Ef0;