PlasmaPy v0.3.1 release

We are excited to announce the release of PlasmaPy version 0.3.1. This release includes two new subpackages: plasmapy.formulary, which consolidates much of the functionality that was formerly in plasmapy.physics, plasmapy.mathematics, and plasmapy.transport; and plasmapy.simulation, which contains our particle tracker and will be the place to consolidate tools related to plasma simulations. These changes are part of an ongoing plan to reorganize PlasmaPy's subpackage structure that will continue in version 0.4. In order to install PlasmaPy, please refer to the installation instructions on PlasmaPy's documentation page.