Constant lat/lon sanity check

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The sanity checker could be configured to detect constant lat/lon - I've 
spotted a couple of these in the SOCAT v3 data.

Note that if BOTH lat and lon are constant then this is fine - it's either a 
mooring or the ship is stationary for a CTD cast or similar. We should only 
detect if one is constant and not the other.

This should only ever be a warning I think. Transects are certainly possible, 
but it's something that should be double-checked.

Original issue reported on code.google.com by [email protected] on 24 Jul 2014 at 11:41