Python Bindings and their required internal code changes

Created by: williamfgc

Enable adios2 bindings for Python 2.7.x, 3.5 with PyBind11 Removed boost.python dependencies Replaced IO class variadic template arguments for vector ("key=value") and map<string,string> {"key", "value"} versions.

Changes to make core headers completely "public" as needed by external bindings: Variable.inl to Variable.tcc VariableCompound.inl now independent (removed adios2/helper/adiosFunctions.h dependency)

To do: Add tests with other Engines

Created by: williamfgc

@chuckatkins I added the nompi portion for the Python bindings. Need changes in cmake. Thanks!

Created by: chuckatkins

Rather than a seperate nompi file, I think just using an ifdef would be more appropriate. I'll make the pr shortly.

Created by: williamfgc

That's fine.