- attaching user command queue failure now throws a `set_default_queue_error` exception
- default_queue() now takes optional const reference of user queue instead of pointer to user queue
- add missing items in documentation
By doing so, the associated default device and context will automatically match the user-provided command queue. It
Guarantees unique default queue/device/context is returned when calling from multiple threads (requires config macro BOOST_COMPUTE_THREAD_SAFE)
Resolves issue #827