Vectorized versions of dual(re,du) and epsilon(d)

Allow convenient creation of dual arrays from two parallel arrays RE and
DU, and extraction of the epsilon part as an array.
Cholesky factorization of arrays of Duals

Cholesky factorization using chol() for the real part and solving the
matrix equation B = M'*U + U'*M for the epsilon part.  This version is
fairly fast, as it calls through to the BLAS for the heavy lifting.

Requires additional work to play nicely with the Base.cholfact!()
infrastructure.