Add a new interp package with interpolation algorithms: constant, piecewise linear and piecewise constant
ADD
interp/README.md
+# Gonum interp [![GoDoc](https://godoc.org/gonum.org/v1/gonum/interp?status.svg)](https://godoc.org/gonum.org/v1/gonum/interp)
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+Package interp is an interpolation package for the Go language.

Add a new interp package with interpolation algorithms: constant, piecewise linear and piecewise constant