[FXC-3191] Added support for gravity

[NasserFlexCompute]

Note

Medium Risk
Touches core unit-system/conversion and solver JSON translation paths; while gravity is optional, incorrect scaling or translation could affect solver setup for cases that enable it.

Overview
Adds optional global gravity support to fluid simulations via a new Gravity model (direction + acceleration magnitude) exposed on Fluid and re-exported in flow360.__init__.

Extends the unit system to include an acceleration dimension (schema/exposed units + Flow360 base/conversion scaling using V_ref^2/L_ref) and updates the solver translator to emit a gravityVector in solver JSON when gravity is configured. Includes new unit/translation tests covering default/custom gravity and SI non-dimensionalization.

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