Create pathtracing library

[ElisaLegnani]

This PR implements classes for path-tracing.

This is a WIP and aims to implement the following classes:

• [x] abstract struct Pigment: represents the color associated to a point on a surface;
• [x] sub-struct UniformPigment: uniform color;
• [x] sub-struct ImagePigment: textured color;
• [x] sub-struct CheckeredPigment: checkered color;
• [x] abstract struct BRDF: represents the BRDF of a material;
• [x] sub-struct DiffuseBRDF: represents the BRDF of an ideal diffusive surface;
• [x] sub-struct SpecularBRDF
• [x] struct Material: represents the union of Pigment and BRDF of a material;
• [x] abstract struct Renderer
• [x] sub-struct OnOffRenderer
• [x] sub-struct FlatRenderer
• [x] sub-struct PathTracer

and to update some existing classes:

• [x] include Material in Shape constructor;
• [x] include Material in HitRecord.

The goal is to make the program become able to:

• [x] create a complex demo image.