<h3>convexHull function</h3>
Returns the convex hull of a geometry. It represents the minimum convex geometry that encloses all geometries within the set.

<h4>Syntax</h4>
<pre>convexHull( a, b )</pre>

<h4>Arguments</h4>
a &rarr; geometry
a &rarr; geometry

<h4>Example</h4>
<pre>  geomToWKT( convexHull( geomFromWKT( 'LINESTRING(3 3 , 4 4 , 4 10)' )) )   &rarr; returns POLYGON((3 3,4 10,4 4,3 3)) </pre>
