Extract the instrument-specific times, for error-checking

[smwoodman]

dbdreader only identifies sensors as 'engineering' or 'science'. Thus, when extracting any science variable, dbdreader uses 'sci_m_present_time' as the timestamp. For instance, for the data for the sensor 'sci_oxy4_oxygen' have 'sci_m_present_time' as the timestamp, rather than 'sci_oxy4_present_time'.

It could be useful to have binary_to_nc extract eg 'sci_oxy4_present_time', and do a check that these times are not meaningfully different from 'sci_m_present_time'