automatically deduce G constant in particle attributes

[ipelupessy]

Is your feature request related to a problem? Please describe. a number of particleset attributes for gravitational systems use either constant.G or nbody_system.G (or generic G), e.g. potential.. the functions currently use a default and you have to specify (everywhere I think) if you use nbody_system

Describe the solution you'd like determine automatically as in https://github.com/amusecode/amuse/commit/fcc16fe3163859789498cfbd1eafe42a74dbc9ee

Additional context its a bit of copy paste, maybe move the derive_G function to amuse.units

[stale[bot]]

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