poski solver should be working. can run out/demo_poski

however, there are some issues with poski.
I can't use matrix structure hints because we init and close poski multiplie
times. if we init and close only once, we'd be able to use the structure hints.

it seems to run alright without the hunts, however, I've had to hard-code
the number of threads it should be using, as I run into a bug with the
code being very slow when I do not give such hints to the threads.

the other downside of not being able to use the matrix thread hints
is that poski should be able to tune a matrix optimally for a
transpose multiply. that might allow SCS to run faster and save memory,
since we wouldn't need to form the transpose. However, I'm not sure
how well that feature performs, as I haven't tested it.