Expose solver attributes to the user

[Padisi]

I think it could be useful for the user to have access to the solver’s attributes when building applications with libMobility. I mean being able to do:

solver = libMobility.NBody(periodicityX='open', periodicityY='open', periodicityZ='open')
solver.setParameters()
solver.initialize( ... )

print(solver.includeAngular)
print(solver.periodicityX)

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Output:

False
'open'

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