vehicle form: when editing a vehicle in declaration mode, only show axle configurations that are allowed in declaration mode

Extend Driver functionality: if no operating point can be found because the vehicle already stopped and no acceleration > 0 can be achieved (e.g. because steep uphill and the gearbox is disengaged), insert a 'halt' action and drive off again

adding testcase for auxiliary types: read out allowed values from schema and make sure the lookup scceeds

refactoring of XML validation: consider schema version number read from document, use single class for validating all vecto-related xml files