Memory leak when downloading beamline data with Databroker 1

[dmgav]

Memory leak was observed at HXN beamline when make_hdf was used from IPython to download large batch of run data (~70 runs). After ~40-50 files the workstation runs out of memory. Data Broker 0.13.3 provides API to clear cache, which is explicitely used by PyXRF. In Data Broker 1.0 cache is expected to be cleared automatically.