Simplify task interface

In order to improve client experience we have to be able to query information
in a simpler way form tasks. This change:

1. Removes the distinction between UfoGpuIface and UfoCpuIface and merges
functionality into the top-level UfoTaskIface.

and

2. Splits and replaces ufo_task_get_structure() by individial calls to
ufo_task_get_num_inputs(), ufo_task_get_num_dimensions() and
ufo_task_get_mode().
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