Equiripple filter improvements

[chipmuenk]

Some improvements for equiripple filter design:

• Implement minimum order estimation for Hilbert & Differentiator types
• Implement check-box for auto grid_density, using grid_density = (lgrid * N)/(2 * bw)
• When manual order is too LARGE, filter design fails (just as Matlab does). This could be solved with an improved algorithm e.g. Emmanouil Z. Psarakis and George V. Moustakides, "A Robust Initialization Scheme for the Remez Exchange Algorithm", IEEE SIGNAL PROCESSING LETTERS, VOL. 10, NO. 1, JANUARY 2003