Add constructors for PDMat from Cholesky or Symmetric.

Allow the chol member to be the upper or lower Cholesky factor.
Modify methods using the chol member to check uplo and dispatch accordingly.
Rewrote direct calls to BLAS.trmm!, etc as calls to A_mul_B! for the Triangular
matrix extracted from the Cholesky type (no copying should take place for these
calls to 'getindex').