Random_shuffle: by using this option the values comprising the subject’s
association matrix are randomly shuffeled using the “random_shuffle” function
implemented in GraphVar (src/calc/helper folder) keeping the degree of positive
and negative weights for the random networks the same as in the original network.
Compared to the “null_model_und_signed” function this option is less
conservative but very fast. Random_shuffle does not need an input for number of iterations.
Null_model_und_sign: uses the “null_model_und_sign” function of the BCT.