Plot 2D Spring

--------------------------------- Tutorial ---------------------------------

'Spring.m' is a class file which generates coordinates of a 2D spring for plotting. A new spring instance can be created by passing 'spring radius' and 'number of coils'. For example:
```
spr = Spring(5, 10);
```
creates a spring instance `spr` with radius of 5 and coils number of 10. This instance gives coordinates of a spring connecting two arbitrary points by passing the coordinates of the two points into 'spr.getSpr(point1, point2)'. For example:
```
[x, y] = spr.getSpr([-1, 5], [4, 2]);
```
returns the coordinates of the spring connecting point (-1, 5) and point (4, 2), and then you could simply visualize the spring by using:
```
plot(x, y);
```
More details can be found in the demo files.

--------------------------------- Demo Files ---------------------------------

--- demo_StaticSpring.m ---

This demo file plots a simple static spring connecting two arbitrary points.

--- demo_MovingSpring.m ---

This demo file makes an animation of three moving points connectted by two different springs.