add "slice" plots

[rileythai]

Like these? Use plotly subplots and just add alot of them so we can have slices. We'll max at like 12 of them or something.

Two ways to implement:

1. two new plot options -- Slice Heatmap & Slice Scatter
2. add as option in plot settings menu with customizable list

List item menu to customize each of the slices and their range.

Max at 10 or so slices???

I think this can use the groupby interface in vaex.