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

[Barton Willis , Wed, 23 Feb 2005 13:49:56 -0600]:
> We should have a project to support various discontinuous 
> functions--unit step and the like.  They should be simplifying
> functions (as is kron_delta).   Writing a simplifying unit_step
> function isn't all that difficult. But you'll discover bugs and
> other weirdnesses in sign and getting integrate to do anything
> with unit_step seems difficult to me.

I have some code that deals with antiderivatives of step functions,
but it may be a bit idiosyncratic and not complete: it did do what I
needed at the time, and not more; and some of the conventions differ
from what is usual in physics.  And it doesn't change integrate but
rather provides a unitstep_integrate that falls back on integrate for
the hard work.  Maybe someone more knowledgable about Maxima could
change it into something more generally useful.

Albert.