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:

  pineappl/pineappl_cli/src/export.rs

  Line 54 in 0ca737b

  if extension == "appl" || extension == "root" {

  calling the function convert_into_fktable in a similar way if feature = "fktable" and raise an error if not(feature = "fktable")