Added sech and csch under ReciprocalHyperbolic class

Definitions of csch, asech and acsch will follow in further commits.
We should either keep the test for all the functions or for none. I was trying to keep my patch in conformity with the existing code. 
As for a reason, I think it merely tests the reflexive property of sech. Though it is obvious in this case, it's not a bad idea to keep the test.