Move the NaturalCoordinate class from utilities to the objects namespace.

[MFraters]

Move the NaturalCoordinate class from utilities to the objects namespace and folder. It seems like a more logical place for it.

[codecov]

Codecov Report

Merging #399 (9f4b024) into main (1169eb8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #399   +/-   ##
=======================================
  Coverage   99.38%   99.38%           
=======================================
  Files          82       83    +1     
  Lines        5041     5041           
=======================================
  Hits         5010     5010           
  Misses         31       31           

Impacted Files	Coverage Δ
source/features/continental_plate.cc	100.00% <ø> (ø)
...es/continental_plate_models/composition/uniform.cc	100.00% <ø> (ø)
...plate_models/grains/random_uniform_distribution.cc	100.00% <ø> (ø)
...eatures/continental_plate_models/grains/uniform.cc	100.00% <ø> (ø)
.../continental_plate_models/temperature/adiabatic.cc	100.00% <ø> (ø)
...res/continental_plate_models/temperature/linear.cc	100.00% <ø> (ø)
...es/continental_plate_models/temperature/uniform.cc	100.00% <ø> (ø)
source/features/mantle_layer.cc	100.00% <ø> (ø)
...eatures/mantle_layer_models/composition/uniform.cc	100.00% <ø> (ø)
...layer_models/grains/random_uniform_distribution.cc	100.00% <ø> (ø)
... and 20 more

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[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.468 ± 0.020 (s=306)	1.468 ± 0.020 (s=310)	-0.4% .. +0.3%
Slab interpolation curved simple none	1.489 ± 0.030 (s=315)	1.485 ± 0.029 (s=293)	-0.8% .. +0.3%
Spherical slab interpolation simple none	1.435 ± 0.014 (s=324)	1.436 ± 0.014 (s=305)	-0.2% .. +0.3%
Slab interpolation simple curved CMS	2.236 ± 0.027 (s=225)	2.239 ± 0.026 (s=180)	-0.3% .. +0.5%
Spherical slab interpolation simple CMS	2.590 ± 0.009 (s=181)	2.590 ± 0.007 (s=169)	-0.1% .. +0.1%
Spherical fault interpolation simple none	1.775 ± 0.011 (s=244)	1.773 ± 0.011 (s=266)	-0.3% .. +0.1%
Cartesian min max surface	3.565 ± 0.110 (s=130)	3.573 ± 0.108 (s=125)	-1.1% .. +1.5%
Spherical min max surface	10.252 ± 0.202 (s=43)	10.303 ± 0.269 (s=47)	-1.2% .. +2.2%