unit_step (was Re: New piecewise package indevelopment)

"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."

I am not redefining unit_step in pw.mac after all.  I tried it and found that I have to do it in lisp.  So I did it in 
lisp with compfile then it worked but then I thought I should let the user use whatever definition is in effect at the 
time pw.mac is loaded or changed to afterwards.  Flexibility is better.  Also I noticed that pwsumify is also not ideal 
for all applications.  There is a one point overlap at the boundaries which I don't know how to fix.  If there is no 
reply then I will consider using my unit_step.lisp but I am very unsure about that.  I may work out rules for diff and 
integrate of signum anyway even if I don't use them.  This is a mess with so many different kinds of unit step 
functions.  I think being able to diff and integrate signum will be a positive step forwards though.

Rich