Modify the topographic-correction figure in the gravity-processing tutorial

[MGomezN]

The topographic-correction.svg figure has some mistakes:

• Water between the geoid and the ellipsoid should be filled with blue
• Air between the ellipsoid and the geoid should be filled with white
• The continent below the ellipsoid is displayed over the topography line

Page that contains the error or needs to be improved (paste link):

Description of the problem/suggestion:

[MGomezN]

My very first use of Inkscape ever!! 🎉 🎉 🎉 🎉
What do you think, @leouieda @santisoler? Is this figure what we want? (not to mention that my vector strokes seem to have been executed in a severe state of drunkenness)
That area that is under the ellipsoid and over the geoid and the topography, I'm not sure if it also needs vertical black lines because it is not "air between the ellipsoid and the geoid"... or it is, but the topography is in the middle...

[leouieda]

Wow @MGomezN that's really good, particularly for a first time with Inkscape! Well done 🙂

I think this is great and it's fine to have the vertical bars there. Particularly since those are the two parts we're ignoring anyway.