r.horizon: Support parallel computing for the raster mode by OpenMP

[cyliang368]

This PR parallelizes r.horizon module by OpenMP and creates parallelization benchmarks. It was tested on my fork repository (cyliang368#10).

[cyliang368]

r.horizon runs in two modes (point mode and raster mode). The raster mode requires calculation across every cell in the raster in the given area, which can be greatly improved by parallelization.

The point mode is multithreading for multi-directions. The point mode is highly serialized, especially when only a point and a direction are specified. The new_point function runs step by step accumulatively, so it is hard to parallelize it. Since it just does a small amount of computations, so I didn't parallelize it (r.horizon for 1 point & 1 direction the point mode is still serial).

The figures below show that running the point mode (1 point, 1 direction) takes around 100 times less time than the raster mode. Parallelization won't significantly benefit this. In conclusion, I will leave it serial now and work on other heavy computing modules.