Be more flexible for annotation

[lldelisle]

This PR is for #1793

• It improves documentation to explicitly indicate the needed metacolumns.
• Do some check on these metacolumns before adding the GenomicRanges in the 'annotation' slot.
• Allow transcript_id to be the place where the transcript ids are originally stored (this is the case for all gtf files) and simply copy the information to the tx_id column before adding this as 'annotation' slot.

[lldelisle]

I can add tests for this if you want.

[timoast]

Thanks @lldelisle !