Guess year as date type

Sometimes we get a time dimension (e.g., called 'time') which is lacking units or any other attributes that could indicate what type of time it is.

If it is an integer type, and on the range 1800 to 2200 or so, guess that it is years.

Guessing may be OK for use in Data Basin where user can override the guess on import; it may have undesirable side effects in other places within `clover` though. 
