* Add/update Python wrappers for nGraph operations.

- NotEqual, OneHot, Power, Sqrt, Relu, Sign, Sin, Sinh, Tan, Subtract, Select, Tanh, Sum, Reduce,
Softmax, ReplaceSlice, Reverse
- Add UT for Relu, Sign, Sin, Sinh, Sqrt, Tan, Tanh,

* Add UT for cases when Cos and Sin are giving incorrect results.

* Alphabetically sorted imports.

* Small refactoring.

- Update docstrings
- Remove unnecesary auxiliary local variable.