Fix operator-- in some of the new iterators

Found in compiler warning:

```
../Type-OpenVolumeMesh/libs/OpenVolumeMesh/src/OpenVolumeMesh/Core/Iterators.cc:1558:18: warning: equality comparison result unused [-Wunused-comparison]
        hf_iter_ == halffaces.end();
```

src/OpenVolumeMesh/Core/Iterators.cc

@@ -1510,7 +1510,7 @@ CellHalfFaceIterImpl& CellHalfFaceIterImpl::operator--() {
     const std::vector<HalfFaceHandle>& halffaces =
         BaseIter::mesh()->cell(ref_handle_).halffaces();
     if (hf_iter_ == halffaces.begin()) {
-        hf_iter_ == halffaces.end();
+        hf_iter_ = halffaces.end();
         --lap_;
         if (lap_ < 0)
             BaseIter::valid(false);
@@ -1555,7 +1555,7 @@ CellFaceIterImpl& CellFaceIterImpl::operator--() {
     const std::vector<HalfFaceHandle>& halffaces =
         BaseIter::mesh()->cell(ref_handle_).halffaces();
     if (hf_iter_ == halffaces.begin()) {
-        hf_iter_ == halffaces.end();
+        hf_iter_ = halffaces.end();
         --lap_;
         if (lap_ < 0)
             BaseIter::valid(false);