Debugging and  fixing issues with QCQP <-> SOCP conversion

Backwards incompatible change: Cholesky returns L instead of L' now
Rework previous commit to avoid backwards incompatible change
Fixes #791: structural arithmatic for empty matrices
Fixes #791: structural arithmatic for empty matrices
jacobian for empty matrices: proper treatment
Fixes #791: structural arithmatic for empty matrices

Reverting changes for MX, since that exposes other bugs  -- see #380
automatic documentation update
automatic casadibot merge