Python bindings: Check whether to-/from-Python converters are registered before registering them

[tswilliams]

Currently, the following type of warning will be emitted if uhal is imported in Python after another Python library that already provides converter functions some of the same classes as uHAL:

python -c "import smash; import uhal"
/usr/lib/python2.7/site-packages/uhal/__init__.py:5: RuntimeWarning: to-Python converter for std::vector<std::string, std::allocator<std::string> > already registered; second conversion method ignored.

The implementation of the Python bindings should be updated to only load to-/from-Python converter functions if converters haven't already been registered for the corresponding class - e.g. as follows (taking MP7/EMP as an example):

    const bpy::converter::registration* reg;

    reg = bpy::converter::registry::query(bpy::type_id< std::vector<bool> >());
    if (reg == NULL or (*reg).m_to_python == NULL) {
        Converter_std_vector_from_list<bool>();
        bpy::to_python_converter < std::vector<bool>, emp::python::Converter_std_vector_to_list<bool> >();
    }