Fix: more robust transfer functions

[fjebaker]

See also #106, since this PR still bangs its head against a number of ceilings.

[fjebaker]

Transfer functions are working well now, bar some anomalous downwards spike around $g^\ast \approx 0.5$ when $r_e > 150$. Unsure what is causing this, but if memory serves it appeared when modifying the Jacobian code. I'll conduct more extensive tests tomorrow.

[codecov-commenter]

Codecov Report

Merging #107 (e3b7811) into main (f18dad9) will decrease coverage by 1.39%. The diff coverage is 69.92%.

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@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   67.08%   65.70%   -1.39%     
==========================================
  Files          53       53              
  Lines        1993     2044      +51     
==========================================
+ Hits         1337     1343       +6     
- Misses        656      701      +45     

Impacted Files	Coverage Δ
src/accretion-geometry/bootstrap.jl	72.72% <0.00%> (ø)
src/image-planes/grids.jl	51.85% <0.00%> (ø)
src/point-functions.jl	45.83% <0.00%> (ø)
src/tracing/configuration.jl	86.66% <ø> (ø)
src/tracing/constraints.jl	100.00% <ø> (ø)
src/tracing/radiative-transfer-problem.jl	0.00% <0.00%> (ø)
...ransfer-functions/cunningham-transfer-functions.jl	62.91% <58.82%> (-15.52%)	:arrow_down:
src/GradusBase/geodesic-solutions.jl	51.11% <73.91%> (-1.39%)	:arrow_down:
src/tracing/precision-solvers.jl	73.97% <87.50%> (-1.75%)	:arrow_down:
src/corona-to-disc/disc-profiles.jl	58.22% <100.00%> (ø)
... and 7 more

... and 1 file with indirect coverage changes

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[fjebaker]

Found the source of the error:

The dip aligns precisely when $\theta = \pi / 2$, which corresponds to rays pointing directly at the coordinate singularity of the BL coordinates. The fix is to just offset by some $\epsilon$: