Add docstrings for pytm model entities

The CLI based development workflow using ./tm.py --list-elements + ./tm.py --describe <element> is not convenient for threat model creation.

An easier and more efficient workflow to work with model entities (this includes custom model entities) makes use of/depends on docstrings.

E.g. when using google docstring format the Actor class (docstring reference) + __init__ method (google format docstring reference) could be documented like

class Actor(Element):
    """An entity usually initiating actions.

    Attributes:
        port (int): Default TCP port for outgoing data flows.
        protocol (str): Default network protocol for outgoing data flows.
        data (list): List of pytm.Data objects in outgoing data flows.
        inputs (list): List of incoming Dataflows.
        outputs (list): List of outgoing Dataflows.
        isAdmin (bool): Indicates if the actor is an administrator.
    """

    port = varInt(-1, doc="Default TCP port for outgoing data flows")
    protocol = varString("", doc="Default network protocol for outgoing data flows")
    data = varData([], doc="pytm.Data object(s) in outgoing data flows")
    inputs = varElements([], doc="incoming Dataflows")
    outputs = varElements([], doc="outgoing Dataflows")
    isAdmin = varBool(False)

    def __init__(self, name, **kwargs):
        """
        Initialize an Actor object.

        Args:
            name (str): The name of the actor.
            **kwargs: Additional keyword arguments.
                port (int): Default TCP port for outgoing data flows.
                protocol (str): Default network protocol for outgoing data flows.
                data (list): List of pytm.Data objects in outgoing data flows.
                inputs (list): List of incoming Dataflows.
                outputs (list): List of outgoing Dataflows.
                isAdmin (bool): Indicates if the actor is an administrator.
        """
        super().__init__(name, **kwargs)
        TM._actors.append(self)

and would result IDE-wise in something like this:

https://github.com/user-attachments/assets/9ede76e1-3689-4906-bd39-b2af7e0333af

Start in pytm/pytm.py with the Actor class and its init method, then identify the other pytm model entity classes covered by the request. Follow the linked Google-style class and function docstring references, and use ./tm.py --list-elements and ./tm.py --describe to verify that the documented entities expose useful descriptions.