backend: change valdiff into a computed attribute
current state: We (re)compute the valdiff attribute and write it to the txt data by running
./vallex-cli --no-sort --output-format txt -o ../NomVallex/Releases/1.0/ scripts transform vallex/scripts/transforms/add_valdiff.py
and then treat it as any other attribute (read it from the data and show it in the correct place in the output).
Turn it into a dynamically computed attribute (so that we could either remove the computed value from all of our txt files, or use it for tracking changes in the data that require revisiting the value of the
The change should be applied on the master branch (and then merged into production).