Implement set_dofs! method for tuple inputs

[mforets]

Examples include set_dofs!(msh, (:u, :θ), (3, 3)) such as a frame element, for which we want the indices to be continuous for the same element instead of

set_dofs!(msh, :u, 3) 
set_dofs!(msh, :θ, 3) 

julia> collect(dofs(msh))
5-element Vector{Dictionaries.Dictionary{Symbol, Vector{Int64}}}:
 {:u = [1, 2, 3], :θ = [16, 17, 18]}
 {:u = [4, 5, 6], :θ = [19, 20, 21]}
 {:u = [7, 8, 9], :θ = [22, 23, 24]}
 {:u = [10, 11, 12], :θ = [25, 26, 27]}
 {:u = [13, 14, 15], :θ = [28, 29, 30]}

would have [1,2,3], [4,5,6] for the first element, etc.