fixing track masking in preprocessing
This fixes an issue where some jointNormVars
were not properly masked.
It was only checked if the values were 0 and not if it was the actual value or if no track was present
this was found while preparing !47 (merged)
These changes affect all preprocessing related to tracks, thus it requires rerunning the preprocessing for DIPS and UMAMI, @alfroch @jvonahne @pgadow
I am not so happy using a hard coded variable name, but didn't another solution without slowing down the code too much