fix: make nested dataclass public in generated python

[MrBlenny]

Currently dataclasses generated in python look like:

class admittance_controller:

    class Params:
        # for detecting if the parameter struct has been updated
        stamp_ = Time()
        ...
        class __Pid:
            rate = 0.005
            class __MapJoints:
                p = 1.0
                i = None
                d = 1.0
            __map_type = __MapJoints
            def add_entry(self, name):
                if not hasattr(self, name):
                    setattr(self, name, self.__map_type())
                return getattr(self, name)
            def get_entry(self, name):
                return getattr(self, name)
        pid = __Pid()

The problem with this is you cannot access the child classes using admittance_controller.Params.__Pid because of the __. This PR changes this so the classes are no longer private.

class admittance_controller:

    class Params:
        # for detecting if the parameter struct has been updated
        stamp_ = Time()
        ...
        class Pid:
            rate = 0.005
            class MapJoints:
                p = 1.0
                i = None
                d = 1.0
            _map_type = MapJoints
            def add_entry(self, name):
                if not hasattr(self, name):
                    setattr(self, name, self.__map_type())
                return getattr(self, name)
            def get_entry(self, name):
                return getattr(self, name)
        pid = Pid()

Given they were private before I don't expect this will cause any backwards compatbility issues.