retip: better input type check/conversion of retip_apply

[ljocha]

retip_apply accepts 'h5' and 'tabular' input types. If it is given an input of 'tabular' type literally, it runs spell.R, otherwise spell_h5.R. This strategy breaks with subtypes of tabular, which revert to h5 not being h5. Technically, the non-h5 version accepts tsv only.

Consequently, the tool must be given either h5, or a tsv explicitely retyped to tabular, otherwise it fails.

I'm not sure whether we can benefit from some automatic type conversion, so I'm leaving the right solution to experts. But it should be made more robust.