References between speed changes and train protection elements

As stated in the forum (1) there is a need to implement references between speed changes and train protection elements (magnets, crocodiles...).

There are two possible directions:

1. From a 'speedChange' to several 'trainProtectionElements', that in some cases provide a securing function in combination with each other
2. From a 'trainProtectionElement' to several 'speedChange's, because of different speed profiles.

Anyway some kind of grouping element for combined 'trainProtectionElement's is needed.

This topic should not be mixed with the speed panel/speed signal discussion. (#173 (closed))

(1) http://www.railml.org/forum/ro/?group=1&id=152