Add density heatmap

[jnumainville]

Add capability to generate a density heatmap chart

This will need to be done in engine, similar to histogram. Perhaps imshowcan be used as the backend, like bar is used as the histogram backend.

• [ ] #608

[jnumainville]

We would need arrays to do this in imshow, but I believe we can do this with heatmap

import plotly.graph_objects as go

fig = go.Figure(data=go.Heatmap(
                    z=[1, 20, 30, 20, 1, 6, 30, 60, 1],
                    x=[0,1,2,0,1,2,0,1,2],
                    y=[0,0,0,1,1,1,2,2,2],
))