DTable TODO/Ideas

Tables.jl interface:
- [x] DTable as a source
- [x] DTable constructors utilizing the interface better if possible - check what is possible and what can be improved
- [x] Schema usage/handling

Table operations:
- [x] select (can be done with map)
- [x] transform (can be done with map)
- [x] groupby (discrete values or function input for continuous values) https://github.com/JuliaParallel/Dagger.jl/pull/275
- [x] per group reductions
- [ ] apply per group
- [x] join (leftjoin and innerjoin done for DTable and input)
- [x] specialized joins for: DTable and DTable input
- [ ] indexing and make it work with other ops? (grouped index already there, but some general indexing would be nice as well)
- [x] shuffle (internal for groupbys and indexing)

Convenience functions:
- [x] length
- [x] column names
- [ ] first, last
- [ ] describe
- [ ] partition information
- [ ] row distribution across partitions

Docs:
- [ ] Best practices on partitioning the table
- [ ] OnlineStats examples once this is merged https://github.com/joshday/OnlineStatsBase.jl/pull/26