[FXC-3191] Added support for gravity

[NasserFlexCompute]

Note

Medium Risk
Introduces a new physical model plus a new core unit dimension and conversion scaling, which could affect other unit conversions and solver JSON generation if mis-scaled or misused.

Overview
Adds a new Gravity model that can be attached to Fluid to apply a global gravitational body force, and exposes it from the top-level flow360 API.

Extends the v2 unit system and conversion pipeline with an acceleration dimension (including flow360_acceleration_unit and base scaling), and updates the solver translator to non-dimensionalize gravity and emit a solver gravityVector.

Adds test coverage for gravity defaults/customization/normalization, unit-system initialization with acceleration, and end-to-end translation/non-dimensionalization in get_solver_json.

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