Updated type name of interpolator field
Remove YieldCurve type; update ZeroCurve

REMOVED
- This will be added back in later. Only need to have a ZeroCurve
object at this time.

UPDATED:
- ZeroCurve has interpolation field rather than Interpolators field.
The former is what is needed to perform interpolations; the latter was
used to generate the Interpolation object and isn’t needed. Need to
extend Interpolation and interpolate() to deal with extrapolation
regime.
- Renamed zerorates -> zeros. Shorter and sweeter