unit_step (was Re: New piecewise package indevelopment)

Actually I have a better idea.  I will leave pwsumify is it was and redefine unit_step when loading pw.mac.  The user is 
then free to use my unit_step or redefine it again anyway they want.

So it will be unit_step(x):=(signum(x)+1)/2;  or whatever else the user want to change it to.  I will put the diff and 
integrate rules on signum, not unit_step, so Maxima will change unit_step to something with signum in it and then apply 
simplification rules to signum.  I think that is the best way.

Rich