Implement ABS bridge for min/max

Some NLP solvers do not support min/max, but they do support absolute values: we should provide a bridge for that use case. 

Idea from https://yetanothermathprogrammingconsultant.blogspot.com/2020/01/how-to-model-yminx1x2.html

Different from #11 and #14: NLP point of view on these functions, not MIP or CP.