Replace tsv2csv.sh with AWK implementation

tsv2csv.awk copied from ipcc-database project:
https://github.com/medea-project/ipcc-database/blob/master/scripts/tsv2csv.awk

I did not notice any significant difference in the export time
after the replacement in this case, which may be even slightly
longer. The new implementation is more consistent though,
covering corner cases as well as the general case supported
by the previous implementation.