initial topography

[MFraters]

There is already #132, but I think I would like to take it in a slightly different direction. The reason for this is that @Djneu and @alanjyu are interested in initial isostatically equilibrated initial topography. This would make for the for the following requirement list

The user needs to be able to select in the world builder file what kind of topography they want.

1. use the highest feature (to match what the world builder prescribes)
2. isostatically equilibrate.

Once we have a good way for the user to input this, the two (or more) different topography outputs can just be models to be chosen. These options can be worked on independently.

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For option 1, we would need to implement the following:

1. create an output property which returns the topography.
2. compute the topography at a certain location by finding the feature with is on top of all the other ones (depth wise) and then find the depth of the top layer in that location.
3. This would probably need on option per feature and/or layer to exclude it from the topography computation. This is because you would not want a sticky air to be counted as topography.

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For option 2, we would need to implement the following:

1. A density plugin system, which can return the density at a given point in a feature. This would take as input the location (relative in the feature), absolute depth and temperature (this means that this plugin system is dependent on the temperture, and it won't be allowed to have a dependency the other way around).
2. a intitial topogarphy model which computes an integral over the features above (and below till the compensation depth?) it, calls the gravity and density plugin systems and computes the topography.
3. We need to think about wether to only let pass the topography value and let the calling program (e.g. ASPECT) deal with what to do with the extra height, or that we should be adjusting the location of the features based on the isostatically adjusted topography.