Fix dataview summary views' div by 0 error

[dgets]

If the usages listing for a particular substance in either of the dataview summary pages is 0 or 1 elements long, there is a divide by 0 error. Best way to handle this would probably be to test the lengths, and if there is a length of 0 or 1 for any of the lists, raise an error flag. This should be checked for in the view, to avoid trying to process that listing. It should also be passed to the template, and if the error flag exists there, that section of the table should not be rendered, or rendered with an appropriate message that the statistic is unavailable.

This is currently blocking #123, and has borked the dataview summary for individual substances, as well.