Enhancement for multi_array resize

[eugvor@…]

Multi_array can have index_bases, it's really great, but resize only works for growing to the higher indexes. Would be nice to put a bit more code to resize it to the lower indexes as well, just add offset during a copy view to transfer data into a new array inside of the resize method. I've made an implementation outside of boost as a separate method. But it simple enough to add this resize downwards as well as upwards into the boost multi_array::resize implementation.