Enabled using custom mask in factorize

factorize.m

-function [B,C,U,S,V] = factorize(M,k)
+function [B,C,U,S,V] = factorize(M,k,Mask)
 %FACTORIZE Summary of this function goes here
 %   Detailed explanation goes here
 
-Mask = isnan(M);
+if ~exist('Mask','var'); Mask = isnan(M); end
 M(Mask) = 0;
 
 [U,S,V] = utils.svd_nsq(M);