Drawable base class

[martinjrobins]

• base class for Axis, Plot1D etc

member variables:

• std::vector<Drawable *> m_children
• m_area: area as ratio of parent
• m_pixels: current area as raw pixels
• m_time_span: maximum time for animation
• m_times: frame times for animation
• m_time: current time for animation

member functions:

• void resize(const std::array<float, 4> &parent_pixels): resize itself and its children
• template <typename Backend> void draw(Backend &backend): draw itself and its children
• add_frame(time): add a new animation frame
• float get_frame_index(): get frame index associated with current time (as a float, could be in-between frames)
• set_time(time): set current animation time
• 'std::string to_svg()`: create a string to represent the object in the SVG format serialization to svg #16

[martinjrobins]

made Drawables no longer templated on Backend. The particular Backend class is only needed for the draw function

[martinjrobins]

got a basic class, closing this for now

[martinjrobins]

re-opening to add features