Allow frequency of recalculation to be set for subsets/joins

[arthur00]

Subsets and joins are O(n)/O(n^2) and can be very demanding on the server. When specifying the data model, there should be a way to say how frequent a subset is updated.

Suggestion: to update every 5 ticks:

@update_frequency(5)

[arthur00]

Another approach is:
@subset(Car, frequency = 5)