ENH: Implement CGF and random methods for GaussianArrayARD.

For GaussianArrayARD: Cumulant generating function using the messages
from parents is now implemented. Also, random samples can be drawn by
using the method random.
TST: Add tests for GaussianArrayARD CGF computation

Tests for GaussianArrayARD's method _compute_cgf_from_parents, which is
used to compute the cumulant generating function.