Add (legacy) FK-table exporter

[cschwan]

This would require roughly the following steps:

• add a file fktable.rs with the function convert_into_fktable doing the work
• add a new code path for the extension .dat here: https://github.com/NNPDF/pineappl/blob/0ca737bf1f7acb6dcee1efc4e58e1b5d44b4bd74/pineappl_cli/src/export.rs#L54 calling the function convert_into_fktable in a similar way if feature = "fktable" and raise an error if not(feature = "fktable")