Fix treatment of “series” option of “initval_file”

This option was treated as a numeric value: it was a hack to ensure that the
name of the dseries object was printed without quotes in the driver file.

However, that would make the JSON output synctically incorrect.

This commits removes the hack, and treats the “series” option as a string
value (in the same way as for the “data” command). A modification of the MATLAB
code will thus be needed to accomodate the change.