Bridge with support for pressure

[rieder]

Bridge uses get_gravity_at_point for each "kicker system" to determine the kick acceleration on each particle (note that in #556 we discussed renaming this to get_acceleration_at_point).

It would be nice if we can find a way to support a get_pressure_at_point function, for example to bridge two SPH codes working with different settings/resolutions.

[stale]

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