#143 Interruptible statistics code

This makes the statistics calculation code interruptible by CTRL+C by checking PyErr_CheckSignals() in all parts of the code that have the potential to take a significant amount of time, and, if it returns true, throwing py::error_already_set(). This is catched by pybind and makes Python see the "KeyboardInterrupt" exception. In the main loop, PyErr_CheckSignals() is only checked once per second to minimize the performance impact, resulting in no measurable increase in runtime on my system.