to_amat returns tuple for DAG but only a matrix for GaussDAG, breaking inheritability

[rebeccaherman1]

graphical_models/graphical_models/classes/dags/gaussdag.py

Line 541 in 30dcfd4

def to_amat(self):

It would be great of GaussDAGs could have node names as well, but even if they don't, this should return a tuple, to match the syntax of DAGs!