Add in Foraging Ratio

[sborstein]

This should be easy to code. I need to think about the best way users can supply data for ease of use. Could also be done hierarchically so users could calculate it for a specific population all the way to species level? This may make sense, but could be problematic theoretically as it would probably be a mean.

[sborstein]

Completed in Electivity function for release 1.0. Function calculates electivity indices and forage ratios.