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/FieldEulerIntegrator.cpp

@@ -112,7 +112,7 @@ FieldImplicitDirectEulerIntegrator::FieldImplicitDirectEulerIntegrator(
   const Array2D< bool > & rhsMask,  // copy 
   const double alpha
 ) : FieldEulerIntegrator(fieldName,physicsModel,feEngine,atc,rhsMask),
-  alpha_(alpha),solver_(NULL)
+  alpha_(alpha),solver_(nullptr)
 {
    rhsMask_(fieldName_,FLUX) = false; // handle laplacian term with stiffness
    const BC_SET & bcs = (atc_->prescribed_data_manager()->bcs(fieldName_))[0];