Address reviews

- `CashBook[NullCurrency] { get; }` will throw an exception
- Revert `Currencies.USD` changes in user facing algorithms
- Improve some documentation
- Revert some format changes
- Adding more asserts for regression test
- Adding new regression tests using a custom fee model which returns
`OrderFee.Zero`
- Adding a non-usd account currency test to the cash book tests
- Adding some unit tests for `NullCurrency` and `OrderFee.Zero`

Algorithm.Python/CustomModelsAlgorithm.py

@@ -101,9 +101,7 @@ class CustomFeeModel(FeeModel):
                   * parameters.Order.AbsoluteQuantity
                   * d.Decimal(0.00001))
         self.algorithm.Log("CustomFeeModel: " + str(fee))
-        return OrderFee(CashAmount(
-            fee,
-            parameters.AccountCurrency))
+        return OrderFee(CashAmount(fee, "USD"))
 
 class CustomSlippageModel:
     def __init__(self, algorithm):