temporary repo to explore combining Arrays of SeriesPair into multicolumn type with similar behavior
julia> using TimeArrays, MarketData
julia> ohlc = TimeArray(op, hi, lo, cl);
julia> ohlc.colnames = ["Open", "High", "Low", "Close"];
julia> ohlc[1]
1x4 Array{Float64,2} 1980-01-03 to 1980-01-03
Open High Low Close
1980-01-03 | 105.76 106.08 103.26 105.22
julia> ohlc[1:2]
2x4 Array{Float64,2} 1980-01-03 to 1980-01-04
Open High Low Close
1980-01-03 | 105.76 106.08 103.26 105.22
1980-01-04 | 105.22 107.08 105.09 106.52
julia> ohlc[[firstday, tenthday]]
2x4 Array{Float64,2} 1980-01-03 to 1980-01-16
Open High Low Close
1980-01-03 | 105.76 106.08 103.26 105.22
1980-01-16 | 111.14 112.9 110.38 111.05
julia> ohlc["Low"][1:2]
2x1 Array{Float64,2} 1980-01-03 to 1980-01-04
Low
1980-01-03 | 103.26
1980-01-04 | 105.09
julia> ohlc["Open", "Close"][1:2]
2x2 Array{Float64,2} 1980-01-03 to 1980-01-04
Open Close
1980-01-03 | 105.76 105.22
1980-01-04 | 105.22 106.52