Fix methods on Model to omit invalid entries.

Methods like `smtk::model::Model::cellsAs<T>()` and
`smtk::model::Model::groupsAs<T>()` would append cursors
that were invalid rather than omitting values not matching
the container type. For example, calling `model->cellsAs<Edges>()`
should only return cells owned by the model that are edges
(as opposed to vertices, faces, or volumes).
Without this patch, invalid `Edge` cursors were being inserted
for each vertex, face, or volume in the model's set of cells.

smtk/model/Model.cxx

@@ -93,6 +93,10 @@ CellEntities Model::cells() const
 {
   CellEntities result;
   ManagerPtr mgr = this->manager();
+  if (!mgr)
+    {
+    return result;
+    }
   EntityRefArrangementOps::appendAllRelations(*this, INCLUDES, result);
   if (result.empty())
     { // We may have a "simple" model that has no arrangements but does have relations.

smtk/model/Model.h

@@ -72,7 +72,14 @@ template<typename T> T Model::cellsAs() const
 {
   // TODO: This could be done more efficiently without a copy.
   CellEntities tmp = this->cells();
-  return T(tmp.begin(), tmp.end());
+  T result;
+  for (CellEntities::iterator it = tmp.begin(); it != tmp.end(); ++it)
+    {
+    typename T::value_type entry(*it);
+    if (entry.isValid())
+      result.insert(result.end(), *it);
+    }
+  return result;
 }
 
 /// Return the top-level (free) groups of this model in a container of the template type.
@@ -80,7 +87,14 @@ template<typename T> T Model::groupsAs() const
 {
   // TODO: This could be done more efficiently without a copy.
   Groups tmp = this->groups();
-  return T(tmp.begin(), tmp.end());
+  T result;
+  for (Groups::iterator it = tmp.begin(); it != tmp.end(); ++it)
+    {
+    typename T::value_type entry(*it);
+    if (entry.isValid())
+      result.insert(result.end(), *it);
+    }
+  return result;
 }
 
 /// Return the child models of this model in a container of the template type.
@@ -88,7 +102,14 @@ template<typename T> T Model::submodelsAs() const
 {
   // TODO: This could be done more efficiently without a copy.
   Models tmp = this->submodels();
-  return T(tmp.begin(), tmp.end());
+  T result;
+  for (Models::iterator it = tmp.begin(); it != tmp.end(); ++it)
+    {
+    typename T::value_type entry(*it);
+    if (entry.isValid())
+      result.insert(result.end(), *it);
+    }
+  return result;
 }
 
 /// Append free cells of this model to the given \a container. Only valid cells are inserted.
@@ -96,7 +117,12 @@ template<typename T> void Model::appendCells(T& container) const
 {
   // TODO: This could be done more efficiently without a copy.
   CellEntities tmp = this->cells();
-  container.insert(container.end(), tmp.begin(), tmp.end());
+  for (CellEntities::iterator it = tmp.begin(); it != tmp.end(); ++it)
+    {
+    typename T::value_type entry(*it);
+    if (entry.isValid())
+      container.insert(container.end(), *it);
+    }
 }
 
 /// Add all the free cells in \a container to this model.