Clarified and aligned PartialOrder semantics to Order semantics: the only allowed values for partialCompare are -1.0, 0.0, 1.0 and NaN.
Shortened the pmin and pmax methods using these assumptions.
Removed the tryLteqv, tryGteqv, tryGt, tryLt that do not have corresponding operators (they can be easily implemented using tryCompare).
Updated GUIDE.md and README.md to include PartialOrder.
Reverted some changes as not to depend on partialCompare being exactly -1.0, 0.0, 1.0.
Merge pull request #293 from denisrosset/topic/partial-order-doc

A few bugfixes for PartialOrder, and update of documentation