Make unstack + rename + join nice

[oxinabox]

Example:

	function wrangle(combined_df, df, time_index, value_field, variates_field)
		reduced_df = DataFrame(
			time_index = time_index,
			values = df[:, value_field],
			variates = map(df[:, variates_field]) do variate
				Symbol(
					spec.value_field, :_, spec.variates_field, :_,
					variate
				)
			end
		)
		unique!(reduced_df, [:time_index, :variates])
		wide_df = unstack(reduced_df, :time_index, :variates, :values)
		combined_df = join(combined_df, wide_df; on=time_index, kind=:inner)
	end

What this actually does is:

• take a categorical column.
• Renaming all the categories
• unstacking them to go from narrow to wide.
• join on time.

[oxinabox]

cf re:join: JuliaData/DataFrames.jl#1333 (comment)