Clustering functions not using spatial index

[dbaston]

I was poking around a bit recently, trying to figure out why the clustering functions were showing roughly N² runtime performance. It turns out that these functions are not correctly using the GEOS C API for creating an STRtree; where the constructor is requesting a node capacity for the tree, the code is instead providing the _entire_ capacity of the tree. This renders the tree useless, hence the N² performance.

I'll commit a fix to use the default JTS node capacity of 10 here.

Some examples of the performance effect, on my laptop:

Clustering a set of 24,000 points, that I was using as a test of the window API, goes from 8800ms to 130ms.

Clustering 1.4 million points, which did not finish before (within the limits of my patience) now takes 13.5s.

[dbaston]

Corrected in trunk at r14530, 2.2 at r14531