Created by: at7
First part of removing deprecated variation code. Remove StrainSlice related code where possible.
Remove deprecated code StrainSlice and StrainSliceAdaptor. Update StrainSlice occurrences in core API to use Variation::StrainSlice Add tests for remaining use of StrainSlice.
Follow up from this PR: Deprecate AlignStrainSlice and IndividualSlice Possibly move MappedSliceContainer and MappedSlice to Variation API?