Add getMeanPerigee to TLEPropagator

Orekit development
1

Hi all,

In TLEPropagator, the perige is recovered using SGP4/SDP4 theory. It can be treat as somewhat mean element (I am not sure).
Currently I need this to do a further analysis.

So I propose to add an public get method.

    public double getMeanPerigee() {
        return perige;
    }

PS: Is that a convenient way to compute ‘mean’ orbit element from osculating orbit.