Check balanced panels are balanced

[nilshg]

Currently when feeding an unbalanced data set into the BalancedPanel constructor, an inscrutable error is thrown as this

    for (row, i) ∈ enumerate(is), (col, t) ∈ enumerate(ts)
        Y[row, col] = only(df[(df[!, id_var] .== i) .& (df[!, t_var] .== t), outcome_var])
    end

throws on only when there's a period for which one of the units does not have an observation. This should be handled with a useful error message before construction of Y is even attempted (likely we need to check setdiff between dates for all units, might be expensive?)