simplify fails for max and other basic functions

[mmckerns]

simplify fails to simplify equations with max, abs and other simple functions:

max([abs(x0 - x2), abs(x1 - x3)]) <= 5
max([abs(x0 - x1), abs(x2 - x3)]) <= 5

Currently, mean, var, and some other basic functions work. Thus, the above should be made to work as well.

[mmckerns]

Side note: This fails to simplify:
x0*x2*x4 + x1*x2*x5+ x0*x3*x6 + x1*x3*x7 == 0.0
However, this succeeds:
x0*x2*x4 + x0*x3*x6 == -(x1*x3*x7 + x1*x2*x5)
Both should work.

Wow... replacing 0.0 with 0 seems to work. c3a6117 fixes this, but not the main issue.

[mmckerns]

abs added to simplify in f2f1633. It was gnarly.

[mmckerns]

split off abs to it's own ticket (#136)