feat: Add some plotting functionality

[Thomas-Christie]

Feature Request

Given how useful it is to visualise GPs for debugging, it would be nice to provide some basic plotting functionality out of the box (e.g. plot_2d_gp, plot_1d_gp). For now, this can be limited to plotting for GPs, but further down the line we may want to include additional functionality for generating plots for the decision_making module.

A nice example can be found in the regression notebook.

[github-actions[bot]]

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