Add contour plot of the full covariance matrix

[st--]

That is, the N x N covariance matrix, not just the marginals in the main plot or the 2x2 bivariate covariance matrix for $x_1$ / $x_2$ on the right-hand side.

To dos:

• fix reactivity w.r.t. resizing
• simply rescale SVG path instead of recomputing on resizing
• fix color scale (should be diverging not sequential)
• clean up UI (hiding full covariance plot)
• draggable sliders for custom resizing?
• support two-finger touch on full covariance plot