Fixed utradedate in STIRFuture() constructor

- Using uindex.spotlag instead of SP[“delay”] to determine utradedate
in STIRFuture() constructor. Closes #14
- Restore vectorisation of Base.string for Currency type. This is
needed for CashFlow method (undos change in
3907b053366a249e80cfb984b483b01e20ef8f3b)