Version 0.7.0

NOTE: This release includes a change with the potential to be subtly breaking, in addition to lots of deprecations.

• Deprecate NA in favor of missing, NAType in favor of Missing, and isna in favor of ismissing, all three from the Missings package (and soon in Base).
• Deprecate PooledDataArray in favor of either CategoricalArray or PooledArray.
• Change the behavior of eltype. Whereas previously we had eltype(::DataArray{T}) = T, we now have eltype(::DataArray{T}) = Union{T,Missing}. That is, the element type is now "correct" in that it matches what one could expect to get when indexing into a DataArray.

When upgrading existing code to use this release, be sure to verify any assumptions your code had been making about the element type of a DataArray. For packages that depend on DataArrays, you may need to consider updating your package and raising your required DataArrays version to 0.7.0. Depending on the severity of the impact this change has on your package, you may also need to upper bound DataArrays at 0.7.0 in past tags.