Change plot argument from `eq` to `things` to accomodate plotting things that require more than one object

[dpanici]

From Daniel:

Idea: to better match the Objectives API, we could change the first argument eq to things, which would normally be the equilibrium but in this case would be the tuple (eq, field). Instead of adding a new kwarg. And this would also be more general for adding new plot quantities in the future.