writer functions do not perform sanity checks + use misleading options for writing tsv

When having dedicated writer functions such as petab.conditions.write_condition_df etc, I would actually expect that they perform some kind of sanity check on the output. Having pretty bare wrappers around pd.to_csv doesn't seem to helpful, especially since the writers use index=True, which is not consistent with the providede spec and, since the readers don't use index_col=0, leaves an Unnamed 0 column in the imported DataFrame.

Start with the dedicated writer entry points mentioned, such as petab.conditions.write_condition_df, and trace their pd.to_csv options alongside the corresponding readers. Compare the output with the PEtab specification and verify that imported DataFrames do not gain an Unnamed 0 column. Done means the writers perform the required sanity checks and use consistent TSV-writing options.