add Classifier pickle file as EnMAP-Box datas source

[Andreas Janz]

Box crashes when adding the attached pkl file.

    qgsApp = QgsApplication([], True)
    qgsApp.initQgis()

    enmapBox = EnMAPBox(None)
    enmapBox.run()
    enmapBox.addSource(source='classifier.pkl')

    qgsApp.exec_()
    qgsApp.exitQgis()

[Fabian Thiel]

Linked to issue #168