gh-36592: Add pull_from_function_field to curves
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For an algebraic curve C, `C.pull_from_function_field()` provides the
inverse map of `C.function()` between the fraction field of the
coordinate ring of C and the abstract function field of C. This
tightens the integration of the two isomorphic fields. Elements of the
fraction field of the coordinate ring of C is represented by elements of
the fraction field of the coordinate ring of the ambient affine or
projective space, for user's convenience.
This is the missing feature as discussed in
https://groups.google.com/g/sage-support/c/Axkaex63f8w/m/wKL0TzmDAwAJ
The problem there can be solved by
```sage
sage: P2.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: f = 2*x^5 - 4*x^3*y*z + x^2*y*z^2 + 2*x*y^3*z + 2*x*y^2*z^2+ y^5
sage: C = Curve(f)
sage: K = C.function(x/y).differential().divisor() # canonical divisor
sage: basis = (-K).basis_function_space()
sage: Basis = [C.pull_from_function_field(f) for f in basis]
sage: phi = C.hom(Basis, P2)
sage: D = phi.image() # conic
sage: D.degree()
2
sage: D
Closed subscheme of Projective Space of dimension 2 over Rational Field
defined by:
x^2 + x*y + 2*y*z
```
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URL: https://github.com/sagemath/sage/pull/36592
Reported by: Kwankyu Lee
Reviewer(s): John H. Palmieri, Kwankyu Lee
