Add code to read legaltag.

Add code to read the legaltag from a file stored in sdms. The file does not need to be open, and does not need to be a ZGY file. To be used by zgytool to propagate the legaltag when copying. Might also be used directly from application code, although that has not been requested yet.

It is technically possible to instead read the tag from an already open file in order to avoid the time spent to open and close it. However, this is more work to implement and/or will break encapsulation. And it would need to be implemented separately for ZGY and VDS.