Issue #239

Added Date Hierarchy filters to CellSamples, Chip Readouts and Plate Readouts.  Also changed Assay_Start_Time and Readout_Start_Time from DateTime fields to Date fields.  The purpose of these fields is really to support selection of data from relevant time frames, for which a date is sufficient.  The exact time is not that useful, will not be routinely entered by users and dropping it makes the UI somewhat simpler.  Please review.