[PythonCAD] Updates for tenth release

[Art Haas]

Hi.

I've been working on line clipping stuff over the last week or two, and
have implemented some routines in the code that will trim segments and
construction lines within a rectangular region of space. These routines
are being used for redrawing the screen and for testing if these objects
appear within some region of space. I've also got a circle-clipping
routine coded up that I found on from searching on Google, and it is a
pretty slick way of testing if a circle is visible in a region and what
parts of the circle are seen. Routines like these will be needed if
quadtrees are used for storing object references because each node in
the quadtree stores the objects that are found in its spatial region.

I've also started trying to write an Autocad dwg format translator.
There are documents about older versions of this file format available
on the internet, so the ones I got from http://www.opendwg.org deal with
the R13/R14 versions of Autocad. I've also found some stuff on older
versions as well. Newer versions of the format are apparently not yet
available, at least to those who don't shell out $$$.

There won't be a release this month of PythonCAD - there hasn't been
enough changes in the code to justify it. It looks like now the next
release will be sometime in September, probably in the later half of the
month.

Art
-- 
Man once surrendering his reason, has no remaining guard against absurdities
the most monstrous, and like a ship without rudder, is the sport of every wind.

-Thomas Jefferson to James Smith, 1822