New package for drawing direction fields with gnuplot

Hello all,

I have created a first draft of drawdf.texi, and have updated drawdf.mac
as well.  Hopefully they can be included in the upcoming release.
They are available at:

http://www.netris.org/~mhw/maxima/drawdf.mac
http://www.netris.org/~mhw/maxima/drawdf.texi
http://www.netris.org/~mhw/maxima/drawdf-demo.html

    Thanks,
      Mark