r.flowaccumulation: Add a new addon that implements the Memory-Efficient Flow Accumulation parallel algorithm (Cho 2023)

[HuidaeCho]

This PR adds a new addon for fast parallel flow accumulation using OpenMP. It implements https://doi.org/10.1016/j.envsoft.2023.105771. Ported from https://github.com/HuidaeCho/mefa.

Benchmark against r.accumulate (28.216s vs. 50.181s) for 30m DEM for Texas on i7-1370P CPU with 64GB RAM

$ time r.accumulate direction=drain accumulation=accum_old
Input direction format assumed to be degrees CCW from East divided by 45
Reading direction map...
 100%
Allocating buffers...
 100%
Accumulating flows iteratively...
 100%
Writing accumulation map...
 100%

real    0m50.181s
user    0m46.074s
sys     0m3.938s

$ time r.flowaccumulation input=fdr output=accum_new 
Using 20 threads for parallel computation
Reading flow direction raster <fdr>...
Flow direction format assumed to be powers of 2 CW from East
 100%
Input time for flow direction: 6.835615 seconds
Accumulating flows...
Compute time for flow accumulation: 3.490350 seconds
Writing flow accumulation raster <accum_new>...
 100%
Output time for flow accumulation: 17.872626 seconds
Total elapsed time: 28.198680 seconds

real    0m28.216s
user    1m14.207s
sys     0m6.489s