Save the valid track flag in a separate group in the preprocessed files

The following discussion from !559 (merged) should be addressed:

• @svanstro started a discussion: (+1 comment)

  Also I was thinking that we could clean some of the masking stuff up by writing the valid track flag as a separate group at each stage of the preprocessing. Including it in the final training files as a separate group would greatly improve the ease of identifying the masked tracks in the final train files