Faster methods for building metadata with subsets of fields

[kescobo]

Might also be good to deprecate the metadata methods since they now conflict with DataFrames.jl (https://github.com/JuliaData/DataFrames.jl/pull/3055)

Eg.

• get(comm) - get all metadata (to replace metadata(comm))
• get(comm, [:field1, :field2]) - get a vector of 2-pls