change truncation in skim file readers

[mabcal]

All the skim file readers were using short.maxint as a maximum storage value instead of ushort.maxint. I changed all readers to truncate at ushort.maxint -1 (the - 1 just to be safe). This will allow values to be twice as high before truncating (around 650 minutes, miles or dollars, instead of around 325)