Convert water content from percent to a fraction in tian2019 composition model

[danieldouglas92]

I ran into this issue a while ago and I fixed it locally and forgot to open a PR, but I ran into it again just now so I am making sure this is addressed. Within the WorldBuilder the water content output by the tian2019 composition model is in % (i.e. 2%), but ASPECT requires a mass/volume fraction and so for this composition feature to be useful this must be converted to a fraction (i.e 0.02). This PR divides the final value by 100 so that when it is fed to geodynamic modeling codes it is a fraction and not a percent.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.208 ± 0.009 (s=366)	1.206 ± 0.009 (s=382)	-0.3% .. +0.0%
Slab interpolation curved simple none	1.219 ± 0.011 (s=368)	1.215 ± 0.007 (s=374)	-0.5% .. -0.1%
Spherical slab interpolation simple none	1.223 ± 0.011 (s=378)	1.224 ± 0.009 (s=360)	-0.1% .. +0.3%
Slab interpolation simple curved CMS	1.255 ± 0.010 (s=341)	1.253 ± 0.012 (s=379)	-0.4% .. +0.1%
Spherical slab interpolation simple CMS	1.637 ± 0.014 (s=258)	1.643 ± 0.014 (s=293)	+0.1% .. +0.6%
Spherical fault interpolation simple none	1.229 ± 0.014 (s=368)	1.231 ± 0.009 (s=366)	-0.1% .. +0.4%
Cartesian min max surface	2.753 ± 0.018 (s=175)	2.741 ± 0.038 (s=155)	-0.8% .. -0.0%
Spherical min max surface	7.716 ± 0.068 (s=60)	7.780 ± 0.292 (s=59)	-0.9% .. +2.6%

[MFraters]

I have no objections against this, but can you add a changelog?

Also, is this not covered by tests, since no tests change?

[danieldouglas92]

@MFraters Yes I'll definitely add a changelog, and there is a test (gwb-dat/water_content_subducting_plate) that I'll fix, when I made the pull request implementing this feature it gave a non-zero water content, but the test output must have been updated since then and it's now giving a 0 water content everywhere for some reason

[danieldouglas92]

The test results were updated in PR #761, but I looked at what they were before this and confirmed that all the water contents are the same as what they were originally (except now being divided by 100).