issues 3710, 3731

expand now recursively flattens any expression wrt mul and multinomials; this is handled at the main expand level, not within the `_eval_expand_foo` level (which is just suppose to perform the basic expansion on a given type of expression)

The coverage of `Pow._eval_expand_multinomial` is now at 100% (less one line that I am not sure that can be reached).