Change BOX3D to be SRID and dimension aware

[robe]

Discussion here

​http://www.postgis.org/pipermail/postgis-devel/2011-November/016216.html

To be followed up after float to double #1328.

As a consequence of this change, the following also needs to be done for backward compatibility.

box3d_extent will be deprecated and possibly removed, though I think that might break some apps removing it. I have one to test that theory.

deprecate box2d (possibly get rid of it). I think there is too much code out there that relies on box2d that getting rid of it is a pipe dream the moment. So we might want to deprecate it now, remove it from the docs, and encourage people to not use it in a non-violent demonstration of disapproval.

1) ST_Extent returns box3d just like ST_3DExtent currently does. 2) Both ST_3DExtent and ST_Extent be changed to strip the SRID of the improved box3d type by default, with an optional argument — keep_srid (defaulted to false) for those like strk who really want it and want to override the behavior to make it true.

All this stuff better not push out the release time beyond our slated February 2012 or I'll be steaming mad (at least mad for a week).