Added linear deformer

src/linear_deformer.cpp

+#include <linear_deformer.hpp>
+#include <vec3.hpp>
+#include <cmath>
+
+linear_deformer::linear_deformer(double maxelong, double z_zero, double up, point3 center)
+: maxelong(maxelong), z_zero(z_zero), up(up), center(center){
+
+}
+
+double linear_deformer::z(double x, double y) const{
+  double r = std::sqrt((center.x()-x)*(center.x()-x) + (center.y()-y)*(center.y()-y));
+  return 2. * up * maxelong * r - up * maxelong + z_zero;
+}

src/linear_deformer.hpp

+#ifndef LINEAR_DEFORMER_HPP_INCLUDED
+#define LINEAR_DEFORMER_HPP_INCLUDED
+
+#include <deformer.hpp>
+#include <vec3.hpp>
+
+class linear_deformer: public deformer{
+public:
+  //up = -1 to deform drum towards positive z
+  linear_deformer(double maxelong, double z_zero, double up, vec3 center);
+  virtual double z(double x, double y) const override;
+
+private:
+  double maxelong;
+  double z_zero;
+  double up;
+  point3 center; //drum center
+};
+
+#endif