Angular input minimization

[matteo-tacchi]

• Remove the first column of the parsing matrix, and simply reconstruct the angular equilibria using the formula \theta = \sgn(\sin\theta)·\arccos(\cos\theta).
• Remove the last index in the plot_ methods: giving only the indices of the sin and cos allows to retrieve (using columns 2 and 3 of the parsing matrix) the index of the corresponding angle, and access to its equilibrium value using the above constructed angle_eq list.

BONUS: currently the default legend is "x_i" for the plot. Maybe we could add an if loop so that if the variable is an angle (which is easily identified by counting the index size in the input) so that the default legend for an angular value is "\theta_i" with i determined in the above instructions. But this is only cosmetics and can wait after paper submission.

[droste89]

Done in 60f1e9c8a084f5b5178fcde1d67fd6b8ef94abc6