fix: improved specificity of read position bias model by estimating the rate of the predominant read position from the ref supporting observations

[johanneskoester]

Description

QC

• The PR contains a test case for the changes or the changes are already covered by an existing test case.
• The documentation at https://github.com/varlociraptor/varlociraptor.github.io is updated in a separate PR to reflect the changes or this is not necessary (e.g. if the change does neither modify the calling grammar nor the behavior or functionalities of Varlociraptor).