Internal data structure.odt: References to Subset026-7 and Subset026-8

[UweSteinkeFromSiemens]

The document lists a lot of data types and variables to be stored.
Most of the data orign or are derived from data types, messages and packets ("ETCS Language") exchanged between track and train as specified in Subset026-7 and -8.

For clarity and better understanding, the references should be added in the document, even though the exact data representations inside the openETCS kernel software may deviate from that defined for the air gap.