Construct B vectors. See Giangrande et al, 2012.
phidp_mod : 2D array
Phi differential phases.
z_mod : 2D array.
Reflectivity, modified as needed.
filt : array
Input filter.
coef : float, optional.
Cost coefficients.
dweight : float, optional.
Weights.
b : matrix
Matrix containing B vectors.