Add piecewiselinear (#2208)

> A tiny library for linear interpolation. O(log(N)) per evaluation for N control points.

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**Please provide package links to:**

- github.com repo: https://github.com/sgreben/piecewiselinear
- godoc.org: https://godoc.org/github.com/sgreben/piecewiselinear
- goreportcard.com: https://goreportcard.com/report/github.com/sgreben/piecewiselinear
- coverage service link [![cover.run](https://cover.run/go/github.com/sgreben/piecewiselinear/.svg?style=flat&tag=golang-1.10)](https://cover.run/go?tag=golang-1.10&repo=github.com%2Fsgreben%2Fpiecewiselinear%2F)

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Thanks for your PR, you're awesome! 👍

README.md

@@ -1113,6 +1113,7 @@ See [go-hardware](https://github.com/rakyll/go-hardware) for a comprehensive lis
 * [ode](https://github.com/ChristopherRabotin/ode) - Ordinary differential equation (ODE) solver which supports extended states and channel-based iteration stop conditions.
 * [orb](https://github.com/paulmach/orb) - 2D geometry types with clipping, GeoJSON and Mapbox Vector Tile support.
 * [pagerank](https://github.com/alixaxel/pagerank) - Weighted PageRank algorithm implemented in Go.
+* [piecewiselinear](https://github.com/sgreben/piecewiselinear) - Tiny linear interpolation library.
 * [PiHex](https://github.com/claygod/PiHex) - Implementation of the "Bailey-Borwein-Plouffe" algorithm for the hexadecimal number Pi.
 * [sparse](https://github.com/james-bowman/sparse) - Go Sparse matrix formats for linear algebra supporting scientific and machine learning applications, compatible with gonum matrix libraries.
 * [stats](https://github.com/montanaflynn/stats) - Statistics package with common functions missing from the Golang standard library.