Add `GFFRecord`

[padix-key]

Currently GFFFile is a container of a complex tuple:

    **seqid**       ``str``                          The ID of the reference sequence
    **source**      ``str``                          Source of the data (e.g. ``Genbank``)
    **type**        ``str``                          Type of the feature (e.g. ``CDS``)
    **start**       ``int``                          Start coordinate of feature on the reference sequence
    **end**         ``int``                          End coordinate of feature on the reference sequence
    **score**       ``float`` or ``None``            Optional score (e.g. an E-value)
    **strand**      ``Location.Strand`` or ``None``  Strand of the feature, ``None`` if feature is not stranded
    **phase**       ``int`` or ``None``              Reading frame shift, ``None`` for non-CDS features
    **attributes**  ``dict``                         Additional properties of the feature

It would be cleaner if this tuple would be replaced by a GFFRecord dataclass instead.