Add example algorithm and test to test decoding in Allen

This MR adds an example Gaudi algorithm: TestVeloClusters that uses data copied to the host (in HostBuffers) to loop over Allen Velo clusters created by the masked clustering algorithm. This can be used as a starting point for subdetector developers to add tests for the decoding of their subsystem. A test has also been added that run the algorithm.

The readme.md in Rec/Allen has been updated with instruction on how to use the example algorithm.

Goes together with Moore!658 (merged)