* Added a cpp UnPackSizePrefixed<struct_name> generated helper function
Missing helper function added to the cpp API generator for unpacking size prefixed structures
* Added generated test files
* Add `NaN` and `Inf` defaults to the C++ generated code.
* Refactoring: add FloatConstantGenerator
* Refactoring-2:
- remove isnan checking for all float/double values
- add most probable implementation of virtual methods of FloatConstantGenerator
* Add conditional (FLATBUFFERS_NAN_DEFAULTS) isnan checking