Resolve "Observation plane cause infinite loop if absorbing = false"

Closes #534 (closed)

If the particle is not absorbed by the ObservationPlane, it will fall upon the plane and contiously trigger the doContinuous process. This can be fixed by ignoring the particles right on the plane.

Using ObservationPlane without absorbe particles allow us to put multiple observation planes in a single run and get laterial distribution of the shower at multiple longitudinal distances.

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