This function calculates the area enclosed by a closed curve C, which can be traveled 'counterclockwise' ONLY, not crossed loops.
It is a simplified version of the Matlab's 'polyarea' function. Which has no restriction on the orientation of the polygon plus other abilities. However it's 40% faster than the matlab version due to its simplicity.
Coordinate points (x,y) are given by the matrix 'C', where: col.1 = x_coords. and col.2 = y_coords.
The calculation method it is based on the Green's Theorem.