Replace interp2d with RegularGridInterpolator

[hackdna]

Fix #210

[GeorgeEfstathiadis]

@hackdna I was actually working on another fix following this guide https://docs.scipy.org/doc/scipy/tutorial/interpolate/interp_transition_guide.html

  # reorder coefficients so that freqs are in ascending order
  order = np.argsort(freqs)
  coefs = coefs[order, :]
  freqs = freqs[order]

  # interpolate coefficients
  ip = interpolate.RectBivariateSpline(range(coefs.shape[1]), freqs, coefs.T, kx = 1, ky = 1)
  coefs_interp = ip(range(coefs.shape[1]), freqs_interp).T

The 2 versions output close enough values (tolerance 1e-10), so we can go with this approach fine