Prevent integer overflow for large grid files in gwb-grid

[gassmoeller]

When visualizing a large grid file with gwb-grid I noticed a broken .vtu output even though the .wb file worked for smaller resolutions. Turns out there is a conversion to int in the output even though the vector type for the output is vtu11::VtkIndexType which resolves to std::int64_t. This PR fixes the problem and I can visualize the .vtu file correctly. Btw: This is not just a problem for outputs with >2 billion points, as the indices and connectivity for the output can reach this threshold much sooner, the model in question for me had 300 million points.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.21%. Comparing base (e8aa93d) to head (06e11d4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #702      +/-   ##
==========================================
- Coverage   98.24%   98.21%   -0.03%     
==========================================
  Files         107      107              
  Lines        7464     7464              
==========================================
- Hits         7333     7331       -2     
- Misses        131      133       +2     

Files	Coverage Δ
source/gwb-grid/main.cc	97.61% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.023 ± 0.034 (s=430)	1.021 ± 0.005 (s=453)	-0.8% .. +0.3%
Slab interpolation curved simple none	1.029 ± 0.004 (s=449)	1.023 ± 0.004 (s=430)	-0.7% .. -0.5%
Spherical slab interpolation simple none	1.173 ± 0.009 (s=386)	1.174 ± 0.007 (s=384)	-0.1% .. +0.2%
Slab interpolation simple curved CMS	1.059 ± 0.005 (s=398)	1.062 ± 0.005 (s=453)	+0.1% .. +0.4%
Spherical slab interpolation simple CMS	1.559 ± 0.011 (s=284)	1.560 ± 0.011 (s=296)	-0.1% .. +0.3%
Spherical fault interpolation simple none	1.178 ± 0.006 (s=380)	1.178 ± 0.009 (s=386)	-0.2% .. +0.1%
Cartesian min max surface	2.304 ± 0.026 (s=196)	2.301 ± 0.017 (s=197)	-0.4% .. +0.2%
Spherical min max surface	7.204 ± 0.014 (s=66)	7.211 ± 0.035 (s=61)	-0.1% .. +0.3%

[MFraters]

odd, I would have assumed int on most system would have been 64 bit, but looking into it it seems that it is only 32 on most systems. Anyway, thanks for the fix!