Make recognition interval and number of threads external parameters

[denis-stepanov]

Currently the parameters are the interval (called SECONDS in advent.py) and the offset between threads (called OFFSET). It makes sense to make offset calculable and specify a number of threads instead (default should be function of the interval), which is a bit more intuitive for a user. Safeguards shall be put in place to avoid division by zero or some unrealistic configurations (like running many more threads than CPU cores available).