add capability for more than 1 missing flag for USGS and add -999999

[kvanwerkhoven]

USGS parquet files contain a large number of values equal to -28316.818359 (-999999 cfs converted to cms). So seems -999999 must be used as a missing flag in addition to -999. The one example location I checked reports "ice" on those dates. Looks like code currently drops data for missing but allows only a single missing flag (-999). We need to add -999999, and probably other combinations of -9 (-9999, -99999). Note that negative values are present and real at tidal and backwater locations so we cannot simply remove anything < -999.