mjo/cone/decomposition.py: new pointed-linspace decomposition

mjo/cone/decomposition.py: new pointed-linspace decomposition

Begin a new experiment with the pointed-linspace decomposition. We
know how to do this with orthogonal projections, but in Sage we must
work over the rationals (and arithmetic is much faster there anyway).
I sketched a quick proof of concept showing that this can be done, and
it's very likely that PPL does exactly the same thing when it
minimizes the generators in Cone().

But just in case, here's an implementation that definitely does what I
think it does.