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();
```
9 jobs for iter-fixes in 4 minutes and 8 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #78196
VS2015
VS2015-32-bit

00:01:16

passed #78195
VS2015
VS2015-64-bit

00:01:27

passed #78194
VS2017
VS2017-32-bit

00:01:30

passed #78193
VS2015
VS2017-64-bit

00:01:22

passed #78190
Linux
clang-c++11

00:04:06

passed #78192
Linux
cppcheck

00:03:32

passed #78197
Linux
doc

00:00:23

passed #78189
Linux
gcc-c++11

00:04:08

passed #78191
Apple
macos-c++11

00:03:09