convenience method to construct elliptic-curve isomorphisms from u,r,s,t

[yyyyx4]

The EllipticCurve_generic class has methods for computing an isomorphism between two given curves as well as computing the codomain of a particular isomorphism (specified by its coefficients $u,r,s,t$).

What it does not currently have is a method for computing an isomorphism from its (co)domain and the coefficients $u,r,s,t$. The best way to construct such an isomorphism is currently to from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism, which is less than convenient. In this patch we add the desired method.

#sd123

[tscrim]

You should also say something about the default values of rst. (Or perhaps this is obvious to an expert?) Otherwise LGTM.

[yyyyx4]

Thanks, done. (I'd say zero is definitely the single obvious choice for $r,s,t$, but it can't hurt to state it explicitly anyway.)

[github-actions]

Documentation preview for this PR (built with commit 89f8531; changes) is ready! 🎉

[tscrim]

Thank you. LGTM.