* remove use of ParameterizedConstant from unit test. Now using Constant instead. Constant is not a templated class.

* move ParameterizedTensorView to NGVM directory

* element_type cleanup

* add json lib 2.1.1

* add json serialization of graph

* cleanup

* enhance unit test

* remove FunctionProvider class and replace with virtual get_function() in Node

* remove json code from element_type

* move serialize to be directly in the ngraph namespace. cleanup header file.

* add cname check to element::operator==

* add using json = nlohmann::json

* ParameterizedTensorView and ElementType cleanup

* change element to_type() to from()

* Done to achieve goal of having our #include directives all start
  with `"ngraph/`, like LLVM does.