Improve brokerage model python wrapper (#7672)

* Improve GetFillModel()

* Add support for other methods

* Add regresion algos and improvements

* Make Python algorithms regression tests

* Improve unit tests

* Minor changes

Algorithm.Python/CustomSettlementModelRegressionAlgorithm.py

@@ -0,0 +1,49 @@
+# QUANTCONNECT.COM - Democratizing Finance, Empowering Individuals.
+# Lean Algorithmic Trading Engine v2.0. Copyright 2014 QuantConnect Corporation.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from AlgorithmImports import *
+from CustomBrokerageModelRegressionAlgorithm import CustomBrokerageModel
+
+### <summary>
+### Regression algorithm to test we can specify a custom settlement model, and override some of its methods
+### </summary>
+class CustomSettlementModelRegressionAlgorithm(QCAlgorithm):
+    def Initialize(self):
+        self.SetStartDate(2013,10,7)
+        self.SetEndDate(2013,10,11)
+        self.SetCash(10000)
+        self.spy = self.AddEquity("SPY", Resolution.Daily)
+        self.SetBrokerageModel(CustomBrokerageModelWithCustomSettlementModel())
+        self.updateRequestSubmitted = False
+
+    def OnData(self, slice):
+        if self.Portfolio.CashBook[Currencies.USD].Amount == 10000:
+            parameters = ApplyFundsSettlementModelParameters(self.Portfolio, self.spy, self.Time, CashAmount(101, Currencies.USD), None)
+            self.spy.SettlementModel.ApplyFunds(parameters)
+
+    def OnEndOfAlgorithm(self):
+        if self.Portfolio.CashBook[Currencies.USD].Amount != 10101:
+            raise Exception(f"It was expected to have 10101 USD in Portfolio, but was {self.Portfolio.CashBook[Currencies.USD].Amount}")
+
+class CustomSettlementModel:
+    def ApplyFunds(self, parameters):
+        currency = parameters.CashAmount.Currency;
+        amount = parameters.CashAmount.Amount
+        parameters.Portfolio.CashBook[currency].AddAmount(amount)
+
+    def Scan(self, parameters):
+        pass
+
+class CustomBrokerageModelWithCustomSettlementModel(CustomBrokerageModel):
+    def GetSettlementModel(self, security):
+        return CustomSettlementModel()