BPMPD Thread Safety

[marip8]

I tried solving an optimization using the BPMPD solver in a multi-threaded context and encountered a segfault deep inside BPMPD. Switching the solver to both OSQP and qpOASES solves the issue, which seems to indicate that some aspect of trajopt with the BPMPD solver is not thread safe. I'm not sure if this comes from the solver library itself or the trajopt_sco wrapper.

I encounter this issue at this version of the repository.

[marip8]

If I had to hazard a guess, this issue may be caused by the couple of static variables in the BPMPD interface library that get used by multiple functions

https://github.com/ros-industrial-consortium/trajopt_ros/blob/b84e4ae30fa658d2911f3e375bb6009dd8ba7084/trajopt_sco/src/bpmpd_interface.cpp#L205-L207

@Levi-Armstrong @mpowelson is there a reason that these variables are static and not a member of the class?

[Levi-Armstrong]

Nope, This was just how it was originally implemented. Change if needed.