Add full HORTON partitioning options support

[JMorado]

This PR adds support for partitioning schemes to accept HORTON's full parameter set.

Changes

• setup_partition_schemes() now accepts dict format for passing scheme-specific parameters
• All schemes now support lmax (default=3) for multipole expansion control
• Iterative schemes (MBIS, Hirshfeld-I, Iterative Stockholder) support maxiter (default=500) and threshold (default=1e-6)
• Becke scheme supports k (default=3) for switching function order

Usage

schemes = {
    "mbis": {
        "lmax": 4,           # Higher multipole expansion
        "maxiter": 1000,     # More iterations
        "threshold": 1e-7,   # Tighter convergence
    },
    "iterative-stockholder": {
        "lmax": 3,
        "maxiter": 500,
        "threshold": 1e-5,   # Looser convergence for speed
    },
    "becke": {
        "lmax": 3,
        "k": 3,              # Becke switching function order
    },
    "hirshfeld": {
        "lmax": 3,
    },
    "hirshfeld-i": {
        "lmax": 3,
        "maxiter": 500,
        "threshold": 1e-6,
    }
}

session.setup_partition_schemes(schemes, proatomdb="proatomdb.h5")

# Old list format still works (uses defaults)
session.setup_partition_schemes(["mbis", "hirshfeld"], proatomdb="proatomdb.h5")

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