Improved min/max; LambertW



I've checked in some code that improves Maxima's handling of
sign(min/max(...)), sign(min/max(...)-min/max(...)).  This isn't complete,
but I thought I'd start with the simple cases.  This already improves
simplification of min/max . I'll have to write some test cases....

I also checked in a very simple LambertW simplifier, using the name
lambertw(...). The idea is to add it to solve sometime, but it would be nice
if we could also add some numerical evaluation routines for it....

         -s