title: Find area of polygon from xyz coordinates
body: <p>I'm trying to use the <code>shapely.geometry.Polygon</code> module to find the area of polygons but it performs all calculations on the <code>xy</code> plane. This is fine for some of my polygons but others have a <code>z</code> dimension too so it's not quite doing what I'd like.</p>
  
  <p>Is there a package which will either give me the area of a planar polygon from <code>xyz</code> coordinates, or alternatively a package or algorithm to rotate the polygon to the <code>xy</code> plane so that i can use <code>shapely.geometry.Polygon().area</code>?</p>
  
  <p>The polygons are represented as a list of tuples in the form <code>[(x1,y1,z1),(x2,y2,z3),...(xn,yn,zn)]</code>.</p>