README.md updated
Added ZeroCurveBRL dataset
Start changing SpotRateCurve to act like a set of SpotRate

. changed function arguments
. removed interp
Checks implemented in SpotRateCurve

. check for length
. check for ascending collection
. check for unique elements

All tests were implemented
NAMESPACE updated
README.md updated
S3method print updated
SpotRateCurve refactored

. [<- add/update a spot rate
. [ returns a numeric rate
. forward rate returns SpotRate
. tests organized
Docs for generic functions added
neighbors moved to a separate file
README.md updated
Implemented interpolation

. CurveInterpolation is a decorator for SpotRateCurve
. linear interpolation
. loglinear interpolation
. flatforward (money market) interpolation
. docs updated