Move the `add_explanations` function to the script `export_to_rdf.py`

This is created as per the review in the PR : #68
Review:
It might be helpful to provide a summary of which column(s) in the row contained unexpected null values. Otherwise users of the script don't know what data needs adding to the source data (the TSV file) to remove/replace the nulls with suitable values.

Review PR #68 and locate the existing add_explanations function, then inspect export_to_rdf.py to determine where it belongs. Done means the function is moved into that script and the unexpected-null explanation, including the affected column summary described in the review, remains covered.

Written by the indexing model from the issue text.