General refactoring and error correction Added: - Increased the quantity of tests and documentation tests - Polynomial derivation and integration Changed: - Generalization of polynomials - Generalization of PID - Generalization of discretization methods - Generalization of transfer functions - Generalization of linear systems - Generalization of polynomial matrices - Generalization of transfer function matices - Generalization of units of measurement - Generalization of discrete transfer functions - Generalization of Bode and polar plots - Generalization of discrete linear systems - Generalization of linear system solvers - The degree of a polynomial now returns an Option, which is None for zero polynomial - Companion matrix is None for zero degree polynomial Fixed; - Error in the calculation of ideal PID transfer function - Error in the calculation of 2x2 matrix eigenvalues - Error in state space Tustin discretization