Robustify online execution of example strategies (when open prices are missing)

[enzbus]

Today's sp500 execution failed because Yahoo Finance didn't update the price of one symbol. DownloadedMarketData canceled the whole last row of returns, since it applies a dropna on it. The online usage parameter should be more aggressive and, in a case like this, use the last available price (yesterday's adjusted close) as provisional open price for today.

[enzbus]

Turns out the symbol changed name (CDAY became DAY) on Jan 31st. Wikipedia page got updated on Feb 2nd. After updating, strategies runner had no issues with the changed universe.