Implement excess topography

[wkearn]

This pull request adds several functions to compute excess topography using a new method based on solving a constrained eikonal equation. This approach is not identical to excesstopography as currently implemented in the MATLAB version of TopoToolbox, which uses morphological filtering, but the eikonal solver is significantly faster.

There are three new API functions, each of which is namespaced with excesstopography_.

• excesstopography_fsm2d computes excess topography with two-dimensionally varying threshold slopes using the fast sweeping method.
• excesstopography_fmm2d does the same using the fast marching method.
• excesstopography_fmm3d uses a three dimensional lithology, as found in TTLEM3D to define the threshold slopes and uses a form of the fast marching method to compute the excess topography.

The fast sweeping method is faster with constant or slowly varying threshold slopes, and it does not require memory be allocated for a priority queue. The fast marching method is faster with quickly varying threshold slopes and requires a priority queue, but it is also guaranteed to complete with a single scan of the DEM.

The names might be slightly misleading: each function writes the constrained topography to its output parameter. If you want to compute the excess topography, you need to subtract that from the original DEM. Packages built on libtopotoolbox should be able to handle this efficiently, and can provide options for returning either the true excess topography or the constrained topography.

In addition, this PR adds an implementation of a heap-based priority queue that is needed by the fast marching method. It has ptrdiff_t keys and float priorities, and it is suitable for other purposes (namely #10).

The tests add a new executable to prevent the random_dem from getting too crowded, but they likewise use property-based tests to ensure that the slopes of the constrained topography are indeed less than the thresholds. excesstopography_fmm3d is run during the tests, but its results are not checked.

[wkearn]

Some CI-revealed issues here:

• [x] The variable length array syntax in pq_create is not supported across compilers, but it is not necessary.
• [x] Formatting issues
• [x] We need to link libm on POSIX systems, but Windows does not have a separate libm.