Using C++17 execution policies

Once we will allow c++17 (around 2027?), we should also use the parallelization provided by the execution policies. See the explanations (and which STL algorithms can benefit from them).

For MeteoIO, this would be useful in the statistical methods (when computing count(), nth_element(), find_first_of()), in the filters (including find_if(), scan()), in the plugins when applying units corrections (transform()) etc