refactor vector tests and add conversion tests

break into smaller pieces, and add tests for WIP constructor functionality
change Polygon to be closed implicitly

now rather than there being an explicit copied vertex at the end identical to
the first, the Polygon-like behaviour is achieved by appending an additional
segment when Polygon.segments is called
move constructor for point lists to MultipointBase

this way lines and polygons can also be constructed from ordered lists of points