SEGYExport doesn't work with other formats than IBM-float

There are no checks to see what DataSampleFormat the original file used, and there are no checks for the format of the input VDS. The default should be to export as integer data when the VDS is U8 or U16 with bias corresponding to signed integer, and to honor any DataSampleFormatCode metadata in the SEGY category and failing that we can peek at the original binary header to see if that indicates using IEEE.