Update for OSQP release 0.6.0

[Bartolomeo Stellato]

A couple of changes required:

• Matrix P needs to be converted (or already) upper triangular to be passed to OSQP
• The setup function now takes workspace as an argument and returns error statuses

[Ed Barnard]

I've released 0.6.0. Had already made most of those changes against a dev version.

[Bartolomeo Stellato]

Thanks a lot!