Don't reinitialize global variables when creating an XPS object

[dennisjlee]

This allows creating multiple NewportXPS objects in one process and accessing them from different threads (each with their own unique socket and XPS object).

[dennisjlee]

My use case was to create three NewportXPS instances and access them from different threads:

• a main instance to send movement commands (main thread)
• an instance which would be used to poll the current stage positions from a thread
• an instance to be used to interrupt moves (used occasionally from a third thread)

When I initially tried this without my patch, each time I made a NewportXPS object, it would create an XPS, which would reset XPS.__nbSockets and XPS.__usedSockets, so each NewportXPS object would have _sid == 0 and would end up trying to communicate using the same socket.

With this patch, the three NewportXPS objects get _sid = 0, _sid = 1, _sid = 2 as expected, and they can all communicate successfully using separate sockets.

[newville]

@dennisjlee Thanks! I'm sort of surprised that I never hit this before ;).