Dcp2Cone canonicalizer for the log determinant atom

`Dcp2Cone.log_det_canon(expr, args)`

- expr
An Expression object

- args
A list of Constraint objects

A cone program constructed from a log determinant atom where the objective function is the sum of the log of the vector D and the constraints consist of requiring the matrix Z to be diagonal and the diagonal Z to equal D, Z to be upper triangular and DZ; t(Z)A to be positive semidefinite, where A is a n by n matrix.