caltechlibrary/dataset: Refactor frame to support a list of objects

The major change is frames now will include an object list and the grid element of a frame is considered depreciated. The object list has proven more useful in the code we write that uses frames it is also more consistent with how Python and R represent a data frame as well. The array of objects remains easy to export to a 2D array if needed. The code support a BUCKET collection has been removed as this data layout has been out of use for more than a year. The eliminated two verbs migrate verb, simplified check and repair as well as the need to specify a layout when initializing a collection. Since removing the BUCKET layout is conceptually a breaking change (it is removing a feature) and we the grid element in a data frame is being depreciated this release is considered a pre-release while we test practical usage.