Feature/compara wga stats

This is a small update on the stats for the whole-genome alignments.

In the first commit, I remove the "method" parameter to the page, since it can be derived from the mlss_id

In the second commit, I add some more columns to the stats table for multiple alignments. There's one nothing I couldn't get working: the header to export the data, and the icons to sort the columns.
I think the code is OK: it is similar to what I've done for gene-trees, but it doesn't show up. In the generated HTML code, things seem to be there (the form for the export, the header, etc)