fcoll/vulcan: add read_all implementation

add an implementation of the read_all operation that uses the two-phase I/O algorithm using even partitioning, i.e. the same base idea that is used by the write_all operation of this component.

In addition to using the 'correct' data partitioning approach for the component, the vulcan read_all implementation also adds some other features that were there for the write_all operations, but not for the (generic) read_all algorithm used by all components so far. Specifically, it can overlap the execution of the I/O phase and the communication phase. The algorithm can also use GPU buffers for aggregation.

Signed-off-by: Edgar Gabriel <[email protected]>

Author: edgargabriel