This function takes as input a new network proposal and checks that the
proposal does not exceed the maximum number of parents for a node, and that
there are no self loops (if self loops have been disallowed).
The proposed network (K-by-q matrix with K segments and q
parent sets).
qmax
Maximum number of parents allowed.
self.loops
Flag indicating whether self loops are allowed.
target
The current target node (only needed to find out which parent
would be the self loop).
fixed.edges
Which edges in the network should be fixed for all
segments (q-by-q matrix with entries 0 for fixed non-edge, 1 for fixed edge,
-1 for non-fixed edge).
Value
Returns TRUE if the proposed move is acceptable, FALSE
otherwise.