Add util for extracting ribosomal transcripts

[rannick]

Module to extract ribosomal RNA from gtf file

PR checklist

• [x] This comment contains a description of changes (with reason).
• [x] If you've fixed a bug or added code that should be tested, add tests!
• [x] If you've added a new tool - have you followed the module conventions in the contribution docs
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[rannick]

@mashehu you have a point. I will check and see if it is simple enough