Add a public method to compute mean state given an osculating state in the sense of Eckstein-Hechler

There is no public method in Eckstein-Hechler propagator that allows getting a mean state given an osculating state in input.

It would be nice to have such a method.
It could be done in the same fashion as the DSST one, see DSSTPropagator.computeMeanState

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