Fix dewpoint formula inconsistency with saturation_vapor_pressure (#3976)

[xtian15]

Summary

Adds a method parameter to dewpoint() allowing users to choose between:

• 'bolton' (default): Original analytical formula - preserves backward compatibility
• 'ambaum': Numerical inversion of Ambaum (2020), consistent with saturation_vapor_pressure()

Problem

As reported in #3976, saturation_vapor_pressure() uses Ambaum (2020) but dewpoint() inverts Bolton (1980). This causes round-trip inconsistency:

dewpoint(saturation_vapor_pressure(T)) != T  # ~0.01-0.1°C error

Solution

Add optional method='ambaum' that numerically inverts the Ambaum formula:

# Default - unchanged behavior
dewpoint(22 * units.hPa)  # → 19.029°C (Bolton)

# New option - thermodynamically consistent
dewpoint(22 * units.hPa, method='ambaum')  # → 19.043°C

# Perfect round-trip with ambaum method
dewpoint(saturation_vapor_pressure(T), method='ambaum') == T  # < 1e-12°C error

Changes

• dewpoint(): Add method parameter with 'bolton' (default) and 'ambaum' options
• _dewpoint_ambaum(): New helper function using scipy.optimize.brentq
• Tests: Add test_dewpoint_ambaum_roundtrip() verifying consistency
• Docstring: Document both methods with examples

Benefits

• ✅ Non-breaking: Default behavior unchanged, all existing tests pass
• ✅ User choice: Those needing consistency can opt-in to method='ambaum'
• ✅ Future-proof: Maintainers can flip the default when ready

Testing

All 228 tests pass (227 existing + 1 new round-trip test).

Addresses #3976

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