NaN counts

Right now, floating-point aggregations return NaNs where there is no data, but integers like counts have no NaN. Datashader's shade function currently treats integer zeros as if they were NaNs, making them transparent, but there should probably be (a) an option not to treat zeros specially, and (b) an option to allow counts to be returned as floating point arrays with NaNs in no-count areas. HoloViews would probably use (b) so that counts can be treated more consistently with other aggregations.