* Add bfloat16 data type in ngraph

* Update on bfloat16 files
- remove uint16_t related functions
- adding/removing const modifier in functions
- default assignment operator
- style conformance

* Add bf16 element type handling in Constant OP

* Update bfloat16 data type implementation
- support nan and infinity cases
- add rounding option when creating bfloat16 from float

* Update the comment for copyright info