Pivot Reports

Enough of a pivot reporting framework to run position and mtm
reports on a set of trades.

Unfortunately mixed in with this commit are some other changes
related to position calcs - namely
- Creation of a HedgingInstrument trait that has a volume
- Replaced SwapHedgeInfo and FuturesHedgeInfo with plain HedgeInfo
- To avoid confusion with FuturesMarket, the terms 'riskMarket' and
  'riskPeriod' are used in hedging (and position reporting).
Add pivot module to sbt
Add samples for each instrument