Change dist resource to use DataField.dist method

The previous dist resource has been moved to ./dims/ for "dimensions"
since it supports arbitrary number of fields (dimensions) to
aggregate on.

Fix #203

Signed-off-by: Byron Ruth <b@devel.io>
Consider moving this line down to line 166 so that `value_labels` is defined closer to where it is first used. It was a bit confusing reviewing the code at the end of this file and having to scroll all the way back up here to find the definition of `value_labels`.