Bug in YTD return % after including existing holdings

[wilsonleong]

Performance YTD calculation is bugged (after including existing holdings / balance brought forward)

Cost brought forward was not taken into account

CalcIRR() steps:

• [x] Check the period (YTD, 1M, 3M, 6M, 1Y, 3Y, 5Y) - None means since inception
• [x] Check if there is previous data for the selected platform, instrument
• [x] If there is previous data, then get the cost brought forward and add to the df
• [ ] Process the cashflows

[wilsonleong]