Vector has alternative value_sources

I needed to modify the role because Vector does no buffering. This
is handled via a parameter now.
Line uses new type PointPoint

...instead of ArrayRefOfPoints
Vector uses PointPoint and coerces from Line
accessors to alternative input private by default

This change was without consequences even for the test script. This
means that they were really only used privately so far.
Merge branch 'private_readers_for_alternatives' into vector_from_pointpoint
Merge branch 'vector_from_pointpoint' into vector_with_alternative_sources
Merge branch 'vector_with_alternative_sources' into towards_0.024