National system type of trainProtectionElement

Andreas Tanner mentioned in the forum thread [http://www.railml.org/forum/ro/index.php?group=1&offset=20&thread=39&id=99] that it would be good to harmonize the definition of the train protection system type: In the timetable equipment element the type offers an enumeration list for various national systems whereas the train protection system of the infrastructure trainProtectionElement is described by the string parameter system and the physical medium.

For railML 2.2 the infrastructure element trainProtectionElement should be enhanced with a new parameter trainProtectionSystem with the values of type tNationalSystemsType. The string parameter system is then no longer needed and should be marked as DEPRECATED.