Added new kick prescriptions

[astro-abhishek]

References for the kick prescriptions via abskickflag = 7 and abskickflag = 8:

[1] Neutron Star Kicks by the Gravitational Tug-boat Mechanism in Asymmetric Supernova Explosions: Progenitor and Explosion Dependence, Janka H.T., 2017, ApJ 837(1):84, arXiv:1611.07562 [astro-ph.HE].
-- Exact implementation matches Equation 2 from Chattaraj et al. 2026a.

[2] New constraints on the Bray conservation-of-momentum natal kick model from multiple distinct observations, Richards S. M., Eldridge J. J., Briel M. M., Stevance H. F., Willcox R., 2022, Published in: Mon.Not.Roy.Astron.Soc. 522 (2023) 3, 3972-3985.

[astro-abhishek]

Here is a comparison of the existing natal kick prescriptions in COSMIC alongside the two new implementations introduced in this PR (kickflag 7 and 8).

Interestingly, Flags 1, 7, and 8 lead to similar looking distributions. If I understood correctly, Flag 1 (the standard COSMIC Maxwellian) gets its bimodal shape via hardcoded $\sigma$ values to differentiate between high velocity Fe core-collapse and low velocity ECSN/USSN.

In contrast, the "physics-based" formulations in Flags 7 and 8 appear to produce this bimodality naturally. This was the conclusion of my paper as well. And the idea behind this PR based on what we discussed at CCA last month.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.27%. Comparing base (8772c07) to head (8319a7b).
⚠️ Report is 176 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/cosmic/src/kick.f	0.00%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #790       +/-   ##
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- Coverage    86.91%   76.27%   -10.63%     
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  Files           40       59       +19     
  Lines        25542    28122     +2580     
  Branches         0      985      +985     
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- Hits         22198    21450      -748     
- Misses        3344     6367     +3023     
- Partials         0      305      +305     

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[katiebreivik]

👍 Thanks @astro-abhishek!!

Sorry for the delay on this!