Allow initial vallue retrieval for parameters.

src/main/java/org/orekit/estimation/Parameter.java

src/main/java/org/orekit/estimation/measurements/Bias.java

...ava/org/orekit/estimation/measurements/GroundStation.java

.../orekit/estimation/leastsquares/BatchLSEstimatorTest.java

...org/orekit/estimation/measurements/GroundStationTest.java

...ls/java/fr/cs/examples/estimation/OrbitDetermination.java

src/tutorials/resources/orbit-determination.in