mjo/cone/isomorphism.py: new module for (rational) cone isomorphism

mjo/cone/isomorphism.py: new module for (rational) cone isomorphism

The is_isomorphic() method for convex cones in Sage kind of does what
I want, but the fact that it does is considered a bug by the author
since it does not capture the notion of isomorphism useful for toric
varieties. Some day that method will be fixed and it will be much less
useful for testing isomorphism in the usual linear-algebraic sense,
which is what I usually want to check.

This commit adds a new module with the beginnings of an isomorphism
test over the rationals. It should work for all convex cones (not just
pointed cones) though it isn't very smart or fast and probably has a
lot of bugs in it at the moment.