ngravs: Distinct gravitational interactions in gadget-2

We discuss an extension of the massively parallel cosmological simulation code gadget-2, which now enables investigation of multiple and distinct gravitational force laws, provided they are dominated by a constant scaling of the Newtonian force. In addition to simplifying investigations of a universally modified force law, the ngravs extension provides a foundation for state-of-the-art collisionless cosmological simulations of exotic gravitational scenarios. We briefly review the algorithms used by gadget-2, and present our extension to multiple gravities, highlighting additional features that facilitate consideration of exotic force laws. We discuss the accuracy and performance of the ngravs extension, both internally and with an unaltered gadget-2, in the relevant operational modes. The ngravs extension is publicly released to the research community.