add IPerspectiveScoreRow for single perspective rows (stratifications:
categorical, classical)
add IDndItem.isInternal for determining the origin of a dnd item
bugfixing
generalize DistributionElement to handle: TablePerspectives,
Perspective, VirtualArray, Histogram as input
box and whiskers plot for table perspectives + double lists
delete average bar -> box and whiskers
fixing cornercase
Merge branch 'develop' of git@github.com:Caleydo/caleydo-dev.git into develop