[T21] Change subsurface logo

[santisoler]

Would be nice to have a better logo for subsurface

[prisae]

This is a very quick go at @Leguark 's pyramid from the readme. It is a screenshot of the same pyramid without text, and then edited in Gimp.

• It would be cool to have a matplotlib-script of it, so we can do subsurface.make_logo(size) (see, e.g., https://matplotlib.org/stable/gallery/misc/logos2.html)
• Miguel had the idea to add a curved surface a the top, instead of a flat surface. This would indicate that the pyramid is "below" the surface, hence subsurface. We can surely add something like this in ASCII.