gh-609: Non-Limber integration of matter power spectra

[Kiyam]

Description

Levin non-limber implementation for GLASS, does the non-Limber integral in comoving distance first and makes use of the geometric approximation

Changelog entry

Added: Method to calculate matter power spectra without the Limber approximation using the Levin method.

Checks

• Is your code passing linting?
• Is your code passing tests?
• Have you added additional tests (if required)?
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[ntessore]

@paddyroddy, I just saw your changes on this — the implementation needs some further investigation before merging, marking as draft.

[paddyroddy]

@paddyroddy, I just saw your changes on this — the implementation needs some further investigation before merging, marking as draft.

I was just linting/typing, but I'll leave what I have for now