implement polygon clipping

[bbell]

following the work performed in D399, one should implement the polygon clipping methods in 2-D and 3-D and make any obvious improvements possible to the existing code (in particular, this means reducing present code redundancy).

system tests should be developed for polygon clipping in the following cases:

2D — convex, concave, self-intersecting, vertex revisited, collinear
3D — ", not coplanar

and exceptions should be thrown where the queries are not working/implemented.

Furthermore, it's worth thinking about how polygons are specified in rasql. Currently, D399 implements a simple representation of [x1,y1,x2,y2,x3,y3] (x/y[/z] coordinates of each polygon vertex are enumerated as one long list). Instead we could adopt ​WKT syntax, e.g. POLYGON ((x1 y1, x2 y2, x3 y3)), or something else more appropriate.