Fixes GetBuyingPowerModel Method of DefaultBrokerageModel (#6215)

`CashBuyingPowerModel`, which reflected on `AlphaStreamBrokerageModel`, a margin-only brokerage.

It also didn't consider the account type, so `InteractiveBrokersBrokerageModel` was using the margin model even if `AccountType.Cash` was selected.

Removes `GetBuyingPowerModel` method from other Brokerage Models when their cases are covered by `DefaultBrokerageModel`

Fixes some typoes in `TradierBrokerageModel`

Fixes unit and regression tests. For the regression tests, we have explicitly set the brokerage model.

Algorithm.Python/BasicSetAccountCurrencyAlgorithm.py

@@ -22,6 +22,7 @@ class BasicSetAccountCurrencyAlgorithm(QCAlgorithm):
 
         self.SetStartDate(2018, 4, 4)  #Set Start Date
         self.SetEndDate(2018, 4, 4)    #Set End Date
+        self.SetBrokerageModel(BrokerageName.GDAX, AccountType.Cash);
         # Before setting any cash or adding a Security call SetAccountCurrency
         self.SetAccountCurrency("EUR")
         self.SetCash(100000)           #Set Strategy Cash