Added the definition of sech

Definition of csch will follow in further commits.
Added the definition of csch

Also made some corrections and changes in sech and added the inverse function
of cosh.
Created a reciprocal class and added tests

1) Put sech and csch in a ReciprocalHyperbolicFunction class
to avoid code duplication
2)Added tests in core/test_args and for some basic functions
Modified tests after resolving merge conflicts
Moved some more functions to the Reciprocal class

Moved rewrite_as_tanh and rewrite_as_coth to RecicprocalHyperbolic
class.
Fixed broken TeX and tests and merged Mathematica's translations

Also did away with redundant tests
Merge pull request #8401 from shivamvats/hyperbolic

Added sech and csch under ReciprocalHyperbolic class