wrong default cutoff_max for union operation

[notestaff]

The cutoff_max for a simple binary operation defaults to the higher cutoff_max of the operands. But for the union operation, the default way to combine the counters is SUM. This causes many kmers to be unexpectedly excluded from the union. The default cutoff for the union operation should be the sum of the cutoff_max values of the operands. Or, maybe the default operation for union should be MAX, as it is for https://docs.python.org/2/library/collections.html#collections.Counter ? But that would cause backwards incompatibility. Either way, maybe add to the docs and the usage message how cutoff_max and cutoff_min for the result are computed? @marekkokot