Collections should have x values in decreasing order

[RadixSeven]

I've run into problems (for example issue #52) with routines that expect the x values in a collection to be in a certain order. I suspect that the compute_initial_inputs routine has this problem. There are likely others in other places in the library. It would be good to ensure that the x values in a collection are all in descending order. That is, collection.x(1) is the largest. The easiest place to do this is in load_collection*.m and in save_collection. But for use of collections created on the fly, it would also be good to make the routine to do the sorting globally callable.