Implement output per cohort

[wtraylor]

Sometimes it would be useful to have the herbivore output resolved not only to the level of HFT but to the level of cohorts.

The struct Fauna::Output::CombinedData currently only has hft_data as a member. In principle, the “output groups” could not only be HFTs but also cohorts. Then there would be a lot of columns in the text table like "bison01", "bison02", etc. It’s not the most elegant solution, but perhaps the easiest. (But compare my thoughts about refactoring output classes in issue #18.)

These variables would be interesting to have by cohort:

• individual_density
• mass_density
• body_fat
• mortality