Adding SetAccountCurrency for backtesting

- Adding new `SetAccountCurrency()` for backtesting. Has to be called
before adding any `Security` or calling `SetCash()`, else will throw.
- Adding new Non account currency unit tests for `CashBuyingPower`,
`SecurityPortfolioModel`, `SecurityMarginModel`,
`SecurityPortfolioManager`, `Future/OptionMarginBuyingPowerModels`
- Adding new C# regression test using `SetAccountCurrency()`, one for
`CashBuyingPowerModel` and one for `SecurityMarginModel`
- Adding new Py and C# basic regression algorithms using
`SetAccountCurrency()`
- `Options` and `Futures` will use not use `AccountCurrency` as quote
Cash.
- `SecurityBenchmark` value will be in account currency

Algorithm.Python/BasicSetAccountCurrencyAlgorithm.py

@@ -0,0 +1,45 @@
+# 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 clr import AddReference
+AddReference("System")
+AddReference("QuantConnect.Algorithm")
+AddReference("QuantConnect.Common")
+
+from System import *
+from QuantConnect import *
+from QuantConnect.Algorithm import *
+
+### <summary>
+### Basic algorithm using SetAccountCurrency
+### </summary>
+class BasicSetAccountCurrencyAlgorithm(QCAlgorithm):
+    def Initialize(self):
+        '''Initialise the data and resolution required, as well as the cash and start-end dates for your algorithm. All algorithms must initialized.'''
+
+        self.SetStartDate(2018, 4, 4)  #Set Start Date
+        self.SetEndDate(2018, 4, 4)    #Set End Date
+        # Before setting any cash or adding a Security call SetAccountCurrency
+        self.SetAccountCurrency("EUR");
+        self.SetCash(100000)           #Set Strategy Cash
+
+        self._btcEur = self.AddCrypto("BTCEUR").Symbol
+
+    def OnData(self, data):
+        '''OnData event is the primary entry point for your algorithm. Each new data point will be pumped in here.
+
+        Arguments:
+            data: Slice object keyed by symbol containing the stock data
+        '''
+        if not self.Portfolio.Invested:
+            self.SetHoldings(self._btcEur, 1)