[boost] [Boost][Multiarray]Move for_each.hpp from examples to boost/multi_array

Hello Everyone, I wish to draw attention to the fact that the for_each header file for multiarrays in boost is not present in the include/boost section of the boost multiarray repository. Also, the header file does not have a namespace defined inside it and so the compiler becomes confused as to which for_each function to follow, the one from stl or this one. The for_each is a great utility for boost multiarrays reducing code size and increasing readability. I would like to what others would like to say about these changes. Making these changes allows contributors to other boost libraries to use that functionality in a proper manner. Thanking everyone, Yours sincerely, Sayan Chaudhuri