Fix parents, domain, codomain and _composition_ isogenies.
Corrects the precedent commit.

I've restored the `meth::_composition_` (it was not the good ticket
 to change this; see #16245), updated its doctest and also restored
 the doctests for `meth::domain()` and `meth::codomain()`.
Corrects stylistic staff in doctests.
Merge branch 'develop' of trac.sagemath.org:sage into magma
fix magma doctest failures
magma error reporting changed
Derivations for rational function fields
is_perfect() for fields
Added copyright header to finite_field_base.pyx

The contents of that header were created automatically from the contents of git
log (every contributor with >10 lines of contribution).
Trac 11474: CremonaDatabase: methods coefficients_and_data(), data_from_coefficients()
Trac 11474: construct elliptic curves using a UniqueFactory
Trac 11474: reorganise documentation a bit
Trac 11474: minor documentation improvements
Trac 11474: add doctest
Better plotting of polyhedra

* Reinstate the polyhedron.plot(projection_direction=...) option
* Fall back to plotting the affine hull if necessary
* Implement affine hull
* Refactor plotting
* Add plots for 1d, 0d polyhedra for completeness.
Make projection objects display as graphics

They are only used in the graphical display, so let them display as
graphics by default if possible. Also, rename Projection.show->plot
for consistency (show = plot + run viewer).
Fix deprecations and doctests
Merge branch 'master' into u/saraedum/ticket/16518
Trac #16322: Magma doctest failures

These doctests currently fail:

{{{
sage -t --long --optional sage,magma
/bulk/home/malb/software/sage/src/sage/symbolic/expression.pyx
sage -t --long --optional sage,magma /bulk/home/malb/software/sage/src/s
age/rings/number_field/number_field.py
sage -t --long --optional sage,magma
/bulk/home/malb/software/sage/src/sage/matrix/benchmark.py
sage -t --long --optional sage,magma /bulk/home/malb/software/sage/src/s
age/rings/polynomial/polynomial_rational_flint.pyx
sage -t --long --optional sage,magma /bulk/home/malb/software/sage/src/s
age/rings/polynomial/multi_polynomial_sequence.py
sage -t --long --optional sage,magma
/bulk/home/malb/software/sage/src/sage/rings/integer_ring.pyx
}}}

URL: http://trac.sagemath.org/16322
Reported by: malb
Ticket author(s): Martin Albrecht
Reviewer(s): Volker Braun
Trac 11474: fix doctest that depended on the extended Cremona database