Complete field lookups

[thclark]

The Dataset.get_files method allows only a very limited set of field lookups.

Implement a complete set like this

• exact
• iexact
• contains
• icontains
• in
• gt
• gte
• lt
• lte
• startswith
• istartswith
• endswith
• iendswith
• range (of integers)
• date
• range (of date)
• year_equals
• year_in... etc
• iso_year
• month
• day
• week
• week_day
• iso_week_day
• quarter
• time
• hour
• minute
• second
• isnull
• regex