Add schematic figure of gradient boosting

[santisoler]

Add a sketch that symbolizes the gradient-boosting process using the synthetic ground survey. Add reference to the figure on the text. Renames the notebook that creates the schematic figures. Only the figure showing the smaller sets of sources is created through the notebook. the final figure was created through Inkscape by adding the observation points and the location of the original sources.

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:robot: :wave: Hi there, human. It looks like you're editing the manuscript. I'll build it for you whenever you make changes and place the resulting PDF at: https://github.com/compgeolab/eql-gradient-boosted/blob/preview-pr123/manuscript.pdf You can see the resulting PDF of the GJI version of the manuscript at: https://github.com/compgeolab/eql-gradient-boosted/blob/preview-pr123/manuscript-gji.pdf

The preview branch preview-pr123 will be deleted when the PR is closed/merged.

[leouieda]

Sketch we were working on for this figure:

[santisoler]

@leouieda I've changed the figure accordingly to the design we discussed.

Please let me know if you would like to change something of it, if the caption looks ok and/or the reference to the figure in the text looks nice.