Add parallelization across models

[ethanwhite]

Will require changes to portalcasting.

[henrykironde]

Portalcasting: https://github.com/weecology/portalcasting

[ethanwhite]

In the process of watching the build as part of implementing #376 I think the easiest choice that would have big performance gains is to parallelize across species within a model type. We'd get a ~20x speed up with what should be fairly minor changes.