Elliptical pair copulas

The default elliptical families are Gaussian and Student-t.

Family	Parameters used in pair models	Typical role
Gaussian	correlation	symmetric dependence without tail dependence
Student-t	correlation , degrees of freedom	symmetric dependence with heavier joint tails

Both base families admit positive and negative association, so automatic selection does not create rotated duplicates for them.

C1 = GaussianCopula(2, 0.6)
C2 = TCopula(5.0, [1.0 0.6; 0.6 1.0])

The Student-t family is numerically validated but remains a performance target because repeated scalar t-CDF and t-quantile evaluations are comparatively expensive.