Add filename property

[PeterNSteinmetz]

Adds the OriginalFileName property to the NlxHeader. Also cleans up private members and moves a useful method to NlxHeader from test.

This is the last in this sequence of commits for a while now. This now provides a much clearer method of parsing the header which works with all extant files (@JuliaSprenger and thus provides that refactoring discussed a while back ;-). It should be more flexible for future possible variations and easier to modify if needed.

If one wanted to squish all of these since the divergence to master there would be much cleaner code to review, however, it would be major changes in one shot.

[zm711]

Thanks Peter. Sam said he would review this when he has some time. So I assigned him to all of these and he will be the main reviewer.

[samuelgarcia]

Hi Peter, I read the long series of PR that depend of the previous ones. I am OK with all changes.

Because of conflict I propose to merge only this last one that should include all of then. In short merging this one should include also: #1562 #1563 #1566 #1567 #1568

I will push the fix myself.

[samuelgarcia]

@zm711 : this is ready to merge on my side.