moved src/flatten.lisp to share/algebra/charsets/ and related changes

Hi Cliff, you wrote:

> Question - do we preserve the nset filename 
> or make this the new set.lisp? 

I guess I'm inclined to keep the same file name.

> Barton, is the version we have in cvs current? 

I copied nset 1.2.03 from Barton's web site and committed it
a couple of weeks ago. It looks like that's still up to date.

> P.S. - I notice the docs for nset mention a Kronecker delta function
> is defined there.  There are lots of physics related problems that
> just loooove to throw in Kronecker deltas - does it make sense/is it
> workable to make Maxima more aware of how to work with delta
> functions for things like integration? 

I'd like to for Maxima to know more about delta functions,
step functions, stuff like that. I've run across these pretty
often in applications (statistics in this case). I suppose
it wouldn't be easy to impart that knowledge, though.

Hope this helps,
Robert Dodier

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