LTR* input sanity check

This PR introduces some additional checks to ensure that there are no malformed annotations used as input which might lead to assertions/segfaults further downstream the LTRdigest workflow:

- LTR retrotransposons must have two long_terminal_repeat child features
- LTR retrotransposons must be longer than 9 bp for protein domain annotation
- LTR retrotransposons must be longer than 10 bp for polypurine tract annotation

Closes #379.