WIP: parallel version of multi_localminmax.hxx

[minhb]

Hi,

I updated the parallel version of the node iteration section in the following functions:

• boost_graph::localMinMaxGraph
• lemon_graph::localMinMaxGraph
• lemon_graph::extendedLocalMinMaxGraph

The test file is test/openmp/test_omp_multi_localminmax.cxx

There are two issues I have:

1. I could not find the test for function: lemon_graph::extendedLocalMinMaxGraph. So, I tested only two left functions.
2. In the parallel region, I want to get rid of break statement. I think, in the parallel region when one thread hits break statement, the others should stop executing their loop iterations. This could save us some useless work.

Although, I would like to make this pull request to discuss about the parallel version of above functions.

Remark: I would like to make a separate pull request for this issue. Unfortunately, the previous commits still stayed in this pull request. So, please consider changes in only two files: test/openmp/test_omp_multi_localminmax.cxx and include/vigra/multi_localminmax.hxx .

Best regards,
Minh

[ukoethe]

This PR cannot be used as is because we abandoned OpenMP in favor of std::thread. But it still contains lots of useful code, so I keep it open for now.