gwb-grid: use threads by default

[tjhei]

I did not benchmark if this is a good choice, but it is certainly faster than 1 thread. I happy to use something simpler (hardware_concurrency should be fine as well).

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (30ae879) 93.55% compared to head (5b4176c) 93.40%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #587      +/-   ##
==========================================
- Coverage   93.55%   93.40%   -0.16%     
==========================================
  Files          92       92              
  Lines        6346     6349       +3     
==========================================
- Hits         5937     5930       -7     
- Misses        409      419      +10     

Files	Coverage Δ
source/gwb-grid/main.cc	46.74% <50.00%> (+0.45%)	⬆️

... and 3 files with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 30ae879...5b4176c. Read the comment docs.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.023 ± 0.004 (s=455)	0.971 ± 0.073 (s=450)	-6.1% .. -3.9%
Slab interpolation curved simple none	1.026 ± 0.004 (s=461)	0.970 ± 0.003 (s=442)	-5.5% .. -5.4%
Spherical slab interpolation simple none	1.260 ± 0.004 (s=381)	1.071 ± 0.005 (s=395)	-15.1% .. -14.9%
Slab interpolation simple curved CMS	1.099 ± 0.002 (s=441)	1.011 ± 0.004 (s=413)	-8.1% .. -8.0%
Spherical slab interpolation simple CMS	1.857 ± 0.007 (s=278)	1.331 ± 0.006 (s=292)	-28.5% .. -28.3%
Spherical fault interpolation simple none	1.269 ± 0.004 (s=400)	1.076 ± 0.008 (s=367)	-15.3% .. -15.1%
Cartesian min max surface	2.383 ± 0.022 (s=194)	2.165 ± 0.022 (s=205)	-9.5% .. -8.9%
Spherical min max surface	7.377 ± 0.271 (s=64)	6.916 ± 0.039 (s=64)	-7.8% .. -4.6%

[gassmoeller]

I am fine with this, and the benchmarks seem to agree. I will let @MFraters take another look.

[MFraters]

Nice! The benchmarks definitely seem to like this. I also like that is show the default when using --help