New price(), currency() methods; EVAL_DATE

- price() method for Cash
- currency() method for Index and Cash types
- global constant EVAL_DATE evaluated to today() on loading packag.
Replaced instances of today() in code with EVAL_DATE
Code ordering change
Added string and show methods for Currency
Vectorise string method for Currency type
CashFlow type introduced; not exported yet

- NEW: CashFlow type, constructors along with show method
- CashFlow constructor for Cash instrument
value() method for DiscountFactor & InterestRate

This accesses the discount factor & rate fields respectively.
Updated Cash methods

- New: rate() method for Cash.
- Update: price() method for Cash type stripped down. Does not need
date parameter as price should be done at trade. If it is done away
from trade date, then it will need an interest rate curve.
- Refactored CashFlow method for Cash type
- Style change to used cash rather than instr as method parameter. Make
method definition easier to read.
New Deposit methods