usage: [n,wp] = ellipord(wp,ws, rp,rs)

Calculate the order for the elliptic filter (discrete)
wp: Cutoff frequency
ws: Stopband edge
rp: decibels of ripple in the passband.
rs: decibels of ripple in the stopband.

References:

- Lamar, Marcus Vinicius, Notas de aula da disciplina TE 456 - Circuitos
Analogicos II, UFPR, 2001/2002.