libdatrwxx [FEATURE]: mseed: add usec field consistency check
On page 124 of "SEED Reference Manual, Standard for the Exchange of Earthquake Data, SEED Format Version 2.4, August, 2012" with respect to  Data Extension Blockette (8 bytes): > field 4: BYTE: µsec has the data start time down to the > microsecond. The SEED format handles down to 100µsecs. This > field is an offset from that value. The recommended value is > from -50 to +49µsecs. At the users option, this value may be > from 0 to +99µsecs.
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