Merge tag 'pybis-1.6.8' into develop

fixed parent/child bugs for samples and datasets. Details see CHANGELOG.md

pybis/src/python/pybis/CHANGELOG.md

-## Changes with 1.6.8
+## Changes with pybis-1.6.8
 
 * fixed bugs with parents and children of both samples and datasets
 * new samples can be defined with parents / children