[WIP] Added the definition of sech

Definitions of csch, asech and acsch will follow in further commits.
On Fri, Nov 14, 2014 at 03:51:28AM -0800, Shivam Vats wrote:
>    How about also moving methods like is_finite, is_real, conjugate, etc to
>    the Function class

Why do you think it's possible?