Sample at 50ms in the IBM Power9 Energy APIs

[tpatki]

Currently, we're sampling every 250ms -- as a result, when we encounter a code that doesn't do much work and takes less than 250ms, energy deltas get reported as zero.

IBM documentation (Section 6.3 and 11.3.2.2) and says that the in-band power sensors get updated every 500us (0.5ms), but our previous correspondence with IBM collaborators recommended sampling at 50ms or slower (>50ms) for reliable data through OPAL.

We'll update the sampling rate in the IBM Port to be 50ms.