[postgis] new Functions/ Function changes

[Dave Blasby]

CVS version has some new OpenGIS functions (and a few changes).

Comming soon is a bunch of testing, then a release (not sure exactly
when).  
After that, I'll go over the OpenGIS spec and see where we are not
compliant.  I'd appreciate other peoples help on that.
Then, its on to doing the complicated spatial operations.  Hopefully
we'll be using JTS since Martin (and vividsolutions) have spent a lot of
time and talent getting the spatial operators/functions robust.


Length2d() is renamed length()
perimeter2d() is renamed to perimeter()

numgeometries(geometry) works on MULTI* types
geometryn(geometry) works on MULTI* types


from section 2.1.5.1
--------------------
startpoint(geometry) :- if geometry is a linestring, return the first
point.  Otherwise, return NULL.

endpoint(geometry) :- if geometry is a linestring, return the last
point.  Otherwise, return NULL.

from section 2.1.9.1/3.2.18.2
--------------------

centroid(geometry) :- if geometry is a polygon (or multipolygon), return
the mathematical centroid (no guaranteed to be on polygon), otherwise
return NULL. I define centroid as the average location of all the points
in the polygon (outer ring only).  For multipolygons, average all the
points on all the outer rings.

from section 3.2.12.2/3.2.17.2
---------------------
isclosed(geometry) :- if geometry is a linestring then returns 
startpoint == endpoint.  If its not a linestring then return NULL.  If
its a multilinestring, return true only if all the sub-linestrings have
startpoint=endpoint.

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