BUG: Incorrect inconsistent_values error on numeric columns

[jpmckinney]

If a numeric column has numbers with 8 digits (date_number), 14 digits (dateTime_number) or 23 digits (dateTime_nsec) and if those digits happen to be valid dates >10% of the time, then an inconsistent_values validation error is incorrectly reported.

Unless these date/time formats are extant (do we know any software that reads or writes CSV dates in this format?), they should be eliminated.

Note that this happily helps solve #105, and it is in fact a significant speed improvement after #103 (like 25% faster with #103).