ENH: support arc edge specification with origin point

- The arc will frequently enclose an angle less than 180 degrees.
  For the case, it is possible to define the arc by its endpoints
  and its centre (origin) point. For example,

      arc 0 1 origin (0 0 0);

  When defined in the way, any discrepancy in the arc radius for the
  endpoints is resolved by adjusting the origin to ensure that the
  average radius is satisfied.

  It is also possible to specify a \em flatness factor as a multiplier
  of the radius. For example,

      arc 0 1 origin 1.1 (0 0 0);

ENH: minor code cleanup for block edges

ENH: expose point appending as polyList::concat