Handle division by zero error with mfi indicator

[jasonl99]

For the mfi indicator:

If no trading days for the period were negative, we'd end up with a divisor of zero, causing with a divide by zero error.

The change here is to set the money_flow_ratio to positive_period_flows (a float) when there are no negative flows for the period

The ratio calculation for money flows over the period is positive_period_flows / negative_period_flows, with both flows representing the sum of the dollar value traded over the days in the period.

While modifying the values, the if statement also checks if positive_period_flows is zero just to short circuit division (since it's unnecessary if the numerator is zero).