rework LTRharvest X-drop code

This PR addresses #320 by rewriting some of the Xdrop code in LTRharvest:
- Fixed possible OBOEs.
- Make the code more readable and understandable.
- Defensive coding to avoid cases in which no extension is required. The failed assertion reported in #320 is no longer triggered. Closes #320.

Please review this code in the context of the *GtSeqabstract* usage. I will adapt gttestdata soon as this PR slightly changes the output of LTRharvest.