Added type conversions for header and footer types to json, python, string to src/common_cpp/Converters

Type conversion now available right the way up to the python layer.

Note that the type conversion goes like (just for my sanity, why is it like that?):

src/common_cpp/Converter -> Can be called inside modules, does not require python
src/common_cpp/Utils/PyObjectWrap -> Wraps everything in python, called from PyWrapModule
src/py_cpp/PyConverter -> Called from python API

each layer does slightly different stuff for different use cases...

This is done in lp:~chris-rogers/maus/1678 and tests pass locally...