StdMem cache flush enhancement

[gvoskuilen]

StandardMem does not have a command for full cache flushes. A similar need is that it would be helpful to be able to flush data to memory and also trigger a write of memory state to file (e.g., for checkpoint or debug snapshot of state). This would be a separate command that triggered a flush of caches followed by a write-out from all memory devices in the system.

Propose adding:
FlushCache -> flushes caches with the same options such as depth as the existing FlushAddr command
CheckpointMemory or a similar name -> flushes caches and causes all memories, scratchpads, etc. to write out their current data values.