Support for non-overlapping amplicons

MinVar now also works with non-overlapping amplicons.

With this update, the program takes the first and the last position that is covered by 20 reads at least. In the previous version, problems were observed on HIV that was sequenced with two non-overlapping amplicons.

For a similar reason also a drop in the coverage caused the premature end of variant calling. This was observed, for example, on HCV samples.