only available returns are cumprod

[flipdazed]

Currently the only returns available are cumulative products.
I couldn't find any alternative to Calculations().create_mult_index() for cumulative sums

Suggest cumulative sum flag when calculating backtest cumulative PnL

[saeedamen]

I think that's a good idea @flipdazed. I had already started to add this functionality to the QuickCharts class. I'll add it to the backtest too, as a flag. I can add it to the BacktestRequest as a flag for the user to set.

[saeedamen]

@flipdazed I've added a cumulative sum flag to BacktestRequests (and added the associated method in Calculations in findatapy.

eg. br.cum_index = 'mult' # 'mult' or 'add'