Bounds check before assigning C-set subpart

[epatters]

The bare minimum fix for #199. It does not add support for rolling back C-set "transactions" but it does at least ensure that the internal state is consistent. For now, this is all I plan to do about this problem.

julia> using Catlab.CategoricalAlgebra.Graphs

julia> g = Graph();

julia> add_vertices!(g, 3)
1:3

julia> add_edge!(g, 1, 4)
ERROR: AssertionError: 0 <= subpart <= cset.nparts.V
[SNIP]

julia> src(g)
1-element Array{Int64,1}:
 1

julia> tgt(g)
1-element Array{Int64,1}:
 0