Openmp multi localminmax

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