Investigate return data type for operators > and <

[matthewhanson]

The > and < operators use the CImg threshold function and return the same datatype as the input data, even though they are equivalent to bytes (0 and 1). Investigate impact of changing return array to uint8.

[akater320]

There could be some problems with INF/NAN propagation. (I know it's used in GeoAlgorithms::indices.)
Round-tripping the CImg through GeoRaster.read(), which converts NAN/INF to nodata(), might get around any problems.