Introduce ArrayTable based on Dictionaries.jl

[andyferris]

This a more DataFrame-like table, where the containers for the columns (or rows) is a Dictionaries.jl AbstractDictionary{Symbol, AbstractArray}.

In the context of future developments:

• It should be possible to define a "static dictionary" kind of thing, like static arrays, where the column names are tracked and typed. This however would be dealt with in Dictionaries.jl not here
• It should be possible to introduce a DictionaryTable where the rows are indexed by some dictionary key, and the columns are dictionaries that share the same keys & tokens. I find I almost always want to be able to look up rows based on some value! (Note - the column indices are an AbstractIndices <: AbstractDictionary and can just be one of the columns... or we could have multiple primary keys, grouped/partitioned tables, etc...).