Add assertion for adding multiple Sweeps to a single SweepAdder object

Christoph Schwarzmeierrequested to merge
single-sweep-adder-assertion into master

When adding multiple Sweeps to a single SweepAdder object (e.g. timeloop.add() << Sweep(A) << Sweep(B);), none of the Sweeps is actually added to the timeloop. Since the user is not informed about this, debugging can be very tedious.

With this MR, these situations are caught and the execution is aborted.

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