This patch implements a lexicographic max/min union of two PWMAFunctions.

The lexmax/lexmin union of two functions is defined as a function defined on

the union of the input domains of both functions, such that when only one of the

functions are defined, it outputs the same as that function, and if both are

defined, it outputs the lexmax/lexmin of the two outputs. On points where

neither function is defined, the union is not defined either.