Modified get_download

I've updated get_download to return a more user friendly data object, a list of matrices (and one list that I'll fix eventually), representing site data, the chronology, other metadata and the count data for the assemblages with depth.

The fix appears to work for both modern assemblages and pollen cores.  I also updated the help file to reflect the changes and improved the example to actually do something useful (plot changes in a pollen taxon over time).