Add tutorial for GNSS orbit determination

Currently, it exists a unit test in Orekit for the GNSS orbit determination. This test works perfectly.

It can be a good enhancement to add a tutorial based on this unit test. Indeed, Orekit dependencies are limited to 2 external libraries (JUnit and Hipparchus). Moreover, an unit test must not be too long to not increase considerably the building time of the project.

There are 2 main motivations for adding an external tutorial:

1. Estimate more than one satellite orbit in order to considerably improve the accuracy. The main drawback is the computation time.
2. Use external tools to have a more user-friendly initialization of the tutorial. Indeed, if many satellite orbits are estimated, the current initialization of the test is limited.