improve xdrop

add test for backward xdrop for perfect matches
adjust comment
fix positions

xdrop positions in ltrharvest_stream corrected
remove zero length xdrop

do not call xdrop with zero length

add assertion to GtSeqabstract to prevent zero length sequences
positions and assertions

there is the possibility for GtSeqabstracts to have zero length now.
I fixed every occurence where they are used with xdrop, but some code
uses GtSeqabstracts not with xdrop. There zero length GtSeqabstracts
happen.
Merge pull request #240 from Garonenur/GtSeqabstract

Gt seqabstract