provide multi_array::swap

[mborgerding]

... or at least make its members protected to allow a subclass to do so.

swap would be substantially the same as the current resize method.

Background: I use a subclass of multi_array in order to resize during assignment to get around the most dagnerous multi_array pitfall. This works, but I'd also like to fix another longstanding inefficiency ( #24 ) while I'm at it. Unfortunately the private data members make it impossible to fix in my subclass (barring a type-punning hack).