Intrinsic to go from octad diagram to stable reduction

[rbommel]

Make this an intrinsic, and if it is, document it.

[rbommel]

With the subspace graph functionality implemented, see issue #2, it should now become feasible to implement this. The strategy is to implement two functions (maybe both intrinsic?):

• a function that takes an octad diagram and returns the subspace graph (already done);
• a function that takes a subspace graph and returns the stable type, which should just be a matter of writing down the list of 42 possible subspace graphs and their corresponding stable type.

Some special care is needed for the hyperelliptic case. Sometimes this shows up as a Ln, sometimes it shows up in the valuation of the twisted cubics. This is currently not recorded in the subspace graph.