Software sweeps in the Platform

[alecandido]

It should be possible to implement software sweeps directly in the Platform, by exposing a Controller method to process the sweepers, and declare which ones will be taken into account.

The return value of the method could be one of the following two alternatives:

1. int, corresponding to just a cut - sweepers more internal than the cut will be handled by the driver, the previous ones will remain Platform's responsibility
2. list[int], essentially a map: for each (parallel) sweeper, it will return the execution order (such that Platform will also reshape the result), flagging those to be executed by the Platform with -1

The first option is the only one required if we want to strictly honor the order requested by the user (i.e. fail if not possible), the second one if we want to allow the driver to sort the sweepers (which actually I'd not recommend...).

It can be implemented in a backward compatible way in both cases, returning 0 or list(range(len(sweepers))) to signal that all the sweepers (and possibly sorting as well) will be handled by the driver.

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I have been tempted to directly implement this in #1088, but I'm not sure how to handle sweepers when there is no single Controller (i.e. as described in #918). So, I postponed the problem.