Adds NaN check in StandardDeviationExecutionModel [py]

[AlexCatarino]

Expected Behavior

Lean modules should not raise unhandled exceptions.

Actual Behavior

The python version of StandardDeviationExecutionModel raised the following exception:

Runtime Error: Trying to dynamically access a method that does not exist throws a TypeError exception. To prevent the exception, ensure each parameter type matches those required by the 'float'>) method. Please checkout the API documentation.
at __init__
self.SMA.Update(time in StandardDeviationExecutionModel.py: line 124
(Open Stacktrace)

which means that in this code block:

        # warmup our indicators by pushing history through the indicators
        history = algorithm.History(symbol, period, resolution)
        if 'close' in history:
            history = history.close.unstack(0).squeeze()
            for time, value in history.iteritems():
                self.SMA.Update(time, value)
                self.STD.Update(time, value)

value was not-a-number is a row.

Potential Solution

Drop the NaN:

history = history.close.unstack(0).squeeze().dropna()

Reproducing the Problem

N/A

Checklist

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