Added .is_isomorphic for rational quaternion algebras

[S17A05]

Added QuaternionAlgebra_ab.is_isomorphic() to conveniently check whether two rational quaternion algebras are isomorphic.

TODO (future work):

• [] Extend isomorphism check to quaternion algebras over number fields.

#sd123

[pjbruin]

Could you add a type check to verify that A is a quaternion algebra?

[S17A05]

Could you add a type check to verify that A is a quaternion algebra?

Added a check to make sure $A$ is a quaternion algebra of the form $A = (a,b)_K$ since .discriminant() depends on this type of quaternion algebra representation - thanks for bringing this up!

[S17A05]

Could you add a type check to verify that A is a quaternion algebra?

Added a check to make sure A is a quaternion algebra of the form A=(a,b)K since .discriminant() depends on this type of quaternion algebra representation - thanks for bringing this up!

Modified the error message to exclude the word rational since the base field of both quaternion algebras is only checked afterwards.

[github-actions]

Documentation preview for this PR (built with commit e3e69c2; changes) is ready! 🎉

[pjbruin]

Looks good to me.