Spin off plasma calculator into separate affiliated package

[namurphy]

Description of improvement

Right now the plasma calculator is installed as part of PlasmaPy. One possibility in the future would be to split off the plasma calculator into a standalone package.

Motivation

In the lead-up to the Python in Heliophysics Community Summer School, work is ongoing to create a heliocloud Python environment that contains all PyHC core packages. A problem that arose was that PlasmaPy's dependency on voila (and consequently tornado) for the plasma calculator ended up conflicting with requirements for other packages that needed web servers. In a sense, this would let us separate the plasma calculator front-end from the plasmapy back-end. Separating the plasma calculator would decrease the dependency footprint of PlasmaPy. In particular, we may be able to drop PlasmaPy's dependency on the Jupyter ecosystem, as well as dependencies dealing with web server stuff like voila and tornado.

Alternatives

In the short term, we can loosen the requirements for voila and other packages.

Implementation

This would probably be a fair bit of work, and thus I'd place it at low priority.