Sparse cube improvement

Thanks for the library, I've used it in a small project and it's only fair I let you know of a few changes I've made to it, they might be useful.

1. Updated the source to better cope with sparse cubes (it doesn't create an array full of empty entries, just works on the original object). It uses a flag, sparsevalue, in the code to capture and indicate this.

2. Added a couple of routines to grab data grouped in certain ways i.e. group entries into arrays where entries in each array have the same values in all dimensions, except one, and that one dimension orders the entries within an array. This is a bit less useful in general maybe, but was useful for things I was doing (grabbing time-series mainly)

Thanks, Martin