Allow for datasets that have time columns at a level more general than date

[levyj]

Some datasets have time elements to them but at a higher level than specific date -- possibly year, quarter, or month. It would be great to be able to include them. This could be done in a number of ways but one would be to cast the period to the beginning, ending, or midpoint date. This would require some human discretion, of course.

The specific example that prompted my suggestion is https://data.cityofchicago.org/Environment-Sustainable-Development/Chicago-Energy-Benchmarking/xq83-jr8c, where the year could fairly be cast to December 31 of that year according to the business owner of the dataset.

This issue is similar to #141 .

[vforgione]

This is an interesting edge case. We're going to start working on a new road map for the application, and this would be something we should keep in consideration as we reevaluate our database design.

Thank you for this!