Add the first draft of the code structure.

[alarshi]

This PR adds a brief documentation of the code structure to address #532 . I am keeping the original comment of what this file should include for reference.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.118 ± 0.009 (s=404)	1.117 ± 0.010 (s=404)	-0.3% .. +0.1%
Slab interpolation curved simple none	1.120 ± 0.011 (s=392)	1.121 ± 0.017 (s=414)	-0.3% .. +0.3%
Spherical slab interpolation simple none	1.099 ± 0.008 (s=413)	1.100 ± 0.009 (s=408)	-0.1% .. +0.2%
Slab interpolation simple curved CMS	1.167 ± 0.010 (s=384)	1.167 ± 0.011 (s=390)	-0.2% .. +0.2%
Spherical slab interpolation simple CMS	1.450 ± 0.015 (s=317)	1.449 ± 0.013 (s=306)	-0.3% .. +0.2%
Spherical fault interpolation simple none	1.101 ± 0.008 (s=400)	1.101 ± 0.008 (s=420)	-0.1% .. +0.2%
Cartesian min max surface	2.544 ± 0.016 (s=195)	2.546 ± 0.017 (s=161)	-0.2% .. +0.3%
Spherical min max surface	7.170 ± 0.085 (s=67)	7.151 ± 0.074 (s=61)	-0.9% .. +0.4%

[MFraters]

Thanks for writing this! A few suggestions for clarifications and a bit of rewording.

[MFraters]

Suggested change

The main file that generates the GWB world is `source/world_builder/world.cc` using the specified parameterizations in a given worldbuilder (`*.wb`) file. The executable `gwb-dat` or `gwb-grid` is then used to query the World on a set of specified points or on a grid, respectively.

The main file that is reponsible for creating and accessing the GWB world is `source/world_builder/world.cc`. When creating a new GWB world it uses the parameterizations from the worldbuilder (`*.wb`) input file to create the world. External programs, such as `gwb-dat` or `gwb-grid` or other software such as ASPECT or SEPRAN, can then use the provided interfaces to ask about certain properties (e.g. temperature, composition) on any location.

[MFraters]

Suggested change

	GWB determines where a given point lies in the World amongst the specified tectonic features. The utility functions to compute the distance of point from a feature are defined in the `source/world_builder/utilities.cc`.
	GWB determines where a given point lies in the World amongst the specified tectonic features. This happens mostly through utility functions, which to compute the distance of point from a feature. These and several other utility functions are defined in the `source/world_builder/utilities.cc`.

[alarshi]

I am closing this since I accidentally push it to the upstream and not my fork repository.