Sequence of ocpTT elements

As discussed in the Forum (http://www.railml.org/forum/rw/?group=2&offset=0&thread=83&id=240), there is a need for a sequence attribute in ocpTT to define the correct order of the ocpTTs. A chronological could be expected but is not explicitly given in the xml.

• new attribute sequence with typoe tCounter for element ocpTT
• attribute is optional for 2.2 to avoid breaking changes
• it will become required for 3.0