Create Area and Centroid nodes

[elbertronnie]

• Create Area and Centroid nodes which can provide the area and centroid of VectorData respectively.
• Create new function perimeter_centroid for Bezier and Subpath that calculates the line integral of position.
• Create new function is_linear which check if the bezier curve is equivalent to a line. Used in the calculation of intersection of two bezier curves where initially a line-like cubic bezier would give the wrong area.