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.