Implement sell buy (Market & Limit) operations

[notAmine]

Hello, I tried to implement the different buy / sell operations. I'm opening this PR for feedback and for help debugging the issue of not being able to submit orders due to the ' {"error":"quantity is invalid."} ' mysterious error!

[Zechtitus]

@notAmine Think I finally figured out what was going wrong with the post
basically the gotcha is that the post needs to have json=data.

here is a code snippet from my code where I am just making my own requests call - adding the auth header and then doing the call. Works for me

   def limit_buy(self, account_id, security_id, price, quantity):
        data = {
            "account_id": str(account_id),
            "quantity": quantity,
            "security_id": str(security_id),
            "order_type": "buy_quantity",
            "order_sub_type": "limit",
            "time_in_force": "day",
            "market_value": float(price),
            "limit_price": float(price)
        }
        try:
            import requests
            url = "https://trade-service.wealthsimple.com/orders"
            session = self.ws.session
            headers = {
                'Authorization': session.headers['Authorization']
            }
            response = requests.post(url, json=data, headers=headers)
            response = response.json()
            return response
        except Exception as e:
            print(e)

in your own code I attempted to give an example of how to fix - think this is what you need

    def place_order(self, order: dict) -> dict:

        """Posts an order (market) to Wealthsimple API
        Parameters
        ----------
        :param order:
            The order dict containing the data to be submitted
        Returns
        -------
        dict
            A dict representing the  submitted order
        """
        url = "{}/orders".format(self.TradeAPI.APIMAIN)
        headers = {
            'Authorization': self.session.headers['Authorization']
        }
        response = requests.post(url, json=order, headers=headers)
        response = response.json()
        return response["results"]

[notAmine]

Thank you @Zechtitus! I moved on to different projects but I hope this code benefits others :)