Update image schema in conceptual intro

[EricThomson]

in Issue #147 I discussed adding a version of the LNP model image with the spike history filter and coupling filter added in. Taking this:

And adding some arrows/text to make it something like the canonical Pillow pic:

But the goal is to shift to a more consistent theme throughout the repo, to present things in a form more like the polished graphics at the readme:

Unfortunately when working on the demos I didn't have the time/skills to pull this off quickly while maintaining the current simplicity of the conceptual intro, so I'm leaving this as an open issue for now, maybe something someone can come back to after the workshop.

[BalzaniEdoardo]

I'll drop a couple of schemas I created for slides I used at the workshop. Some graphics similar to this would work, an adaptation of this would work