numeric terms + refdate = date terms

Using as.spotratecurve numeric method together with refdate yields date
terms according to rates’ calendar
Created units.spotratecurve
Implemented interpolation tests

- interpolation tests
- flat forward adjustments to use curve’s methods
Implemented digits in print function
as.data.frame.spotratecurve + terms arguments

- as.data.frame supports terms arguments
- refdate method
- subcurve method
Implemented interp method

- interp method
- interp<- method
Changed to use the new spotratecurve class