[filter_matrix.c] improved code and comments about buried columns
[filter_matrix.c] improved code and comments about buried columns

mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs