Add MultipoleExpansionPotential for multipole expansion of arbitrary densities

[jobovy]

Summary

• Adds MultipoleExpansionPotential, a new potential class that decomposes a given density function into real spherical harmonics on a radial grid and computes the gravitational potential via classical multipole integrals (galaxiesbook.org Ch. 12.3, eq. 12.76 & 12.79)
• Supports spherical, axisymmetric, and general (non-axisymmetric) density profiles via the symmetry parameter; accepts density as a callable (1/2/3 args) with or without astropy units, or a galpy Potential instance
• Analytical forces via SphericalHarmonicPotentialMixin providing the spherical-to-cylindrical force chain rule (_Rforce, _zforce, _phitorque); radial force derivatives include spline derivative corrections for inner/outer integrals, ensuring forces are the true negative gradient of the interpolated potential
• Second derivatives computed numerically via NumericalPotentialDerivativesMixin

Refactoring

• Extract shared spherical harmonics utilities (compute_legendre, sph_harm_normalization) from SCFPotential into galpy/util/special.py
• Extract shared spherical-to-cylindrical force chain rule from SCFPotential into SphericalHarmonicPotentialMixin; SCFPotential now inherits from the mixin, removing duplicated code

Other changes

• Add MultipoleExpansionPotential to estimateDeltaStaeckel special-case handling (like SCFPotential) to avoid negative delta2 from numerical second derivatives
• Add adiabatic fallback for MultipoleExpansionPotential in orbit analytic parameter tests (ecc, rperi, rap, zmax)
• Exclude SphericalHarmonicPotentialMixin from orbit/potential test lists
• Add tolerances for MultipoleExpansionPotential in test_potential.py (Poisson, 2nd derivatives)

Test plan

• 34 tests in tests/test_MultipoleExpansionPotential.py covering spherical (Hernquist), axisymmetric (MiyamotoNagai), SCF cross-validation, density reconstruction, normalization, isNonAxi flag, input variants (1/2/3-arg callable, Potential instance), analytical forces, and edge cases (r=0, infinity, phi=None)
• Astropy unit density tests (1/2/3-arg) in test_quantity.py::test_potential_paramunits
• All 50 existing SCF tests pass after mixin refactoring
• All test_potential.py and test_orbit.py tests pass for MultipoleExpansionPotential

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.49%. Comparing base (a1150fa) to head (805c485).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
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- Coverage   99.91%   98.49%   -1.42%     
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  Files         215      219       +4     
  Lines       30920    33509    +2589     
  Branches      618      662      +44     
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+ Hits        30893    33005    +2112     
- Misses         27      504     +477     

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