allow passing any matrix as parameter to encoding process

changelog

• add --encoding-method to komodo prove
• pass the encoding matrix to encode and fec::encode instead of k and n, these two parameters can be extracted without further check by looking at the shape of the encoding matrix
• the global recoding vector is now extracted from the encoding matrix instead of recomputing it (see new Matrix::get_col implementation)
• linalg and Matrix::{random, vandermonde} have been made public (see new Matrix::random implementation)
• the computation of Matrix::vandermonde has been optimized