Add density structure to world builder

[Djneu]

Here is the initial try at this with an added continental plate model. So far, in the uniform model you give the number of compositions and then a list of densities for each composition.

For the calculation, I removed the max min depth, as it doesn't seem necessary when the density only depends on the composition. But maybe that will make things easier later on with multiple features?

[Djneu]

I've updated the density files and added a test (hopefully in the correct location). I wasn't able to set densities to nan initially, as it would hang during runtime with the current line of code. Is there a different way to set this since densities is a vector and not a double?

[MFraters]

Sorry for the late review. I think this is almost ready.

I don't think we should always output all the available properties, but make them parameters in the .dat file instead. Something like # density = true.

I am doubting a bit whether to include the density interface in world.h. The idea is to move to the properties interface, but having a direct interace can be really nice, even though it might not be as efficient and scalable...

There is currently a merge conflict, could you rembase?