Cvxpy

Speed ups for QP canonicalization.

Adds FICO Xpress interface and bug fixes.

New features:
* Support function atom (PR 860)
* Scalar product atom (PR 910)

Bug fixes and enhancements:
* Fix formatting of MOSEK cone programs with exponential cones (PR 862)
* Allow warm-starting diffcp (PR 867)
* Default to conic solvers for LPs (PR 871)
* Restart SCS without AA when solution inaccurate (PR 876)
* Fix bugs in detection of monotone real functions for DQCP (PR 909)
* Fix bugs in conic dual variable recovery (PR 910)

Removes some circular dependencies that caused issues on some platforms.

Includes a bug fix for DGP problems. Before this version, DGP problems using
the `quad_over_lin` atom were incorrectly canonicalized.