Enhance sheet writers and zipped portfolio writer to optionally save multi-trade positions by emitting an individual row
for each trade rather than one row for each position. The existing MasterPortfolioWriter is already able to reconstruct
multi-trade positions that were exported in this way, as long as the merge flag is set.
Allow the portfolio saver tool to handle multi-trade positions by optionally emitting one row per trade (-t option)
rather than emitting one row per position as usual. The portfolio loader tool will be able to load multi-trade
positions that were exported in this way, with its soon-to-be-added merge positions option.
Add merge positions (-m) and keep current positions (-k) options to portfolio loader. Merge positions is used to combine
multiple positions in the same security and within the same portfolio node into a single, position that contains all the
original positions' trades. Keep current positions merges the loaded portfolio contents with the contents of latest
existing version rather than starting a new version from scratch.