Add "type" getter for ComparableMeasurement

When dealing with heterogeneous observations in orbit determination for example, it would be nice to be able to easily (especially with the Python wrapper) retrieve the underlying type of measurements when manipulating the instances of EstimatedMeasurement. To achieve this, a "getter" returning a String could be added to the interface ObservableMeasurement or ComparableMeasurement. Tentative implementation:

default String getMeasurementType() {
        return this.getClass().getSimpleName();
    }