LPAbstractDictionary: Add objective_name method

[mkoeppe]

This ticket completes the LPAbstractDictionary interface by providing an objective_name accessor.

CC: @uduse @novoselt @mkoeppe @pgxiao

Component: linear programming

Author: Aedi Wang

Branch/Commit: 288e6f7

Reviewer: Andrey Novoseltsev

Issue created by migration from https://trac.sagemath.org/ticket/20874

[mkoeppe]

Branch: u/mkoeppe/lpabstractdictionary_add_objective_name_method

[mkoeppe]

New commits:

6cad314	add objective_name() to dictionary interface
288e6f7	Doc fix

[mkoeppe]

Commit: 288e6f7

[novoselt]

Reviewer: Andrey Novoseltsev

[mkoeppe]

comment:4

Thanks for reviewing!

[vbraun]

Changed branch from u/mkoeppe/lpabstractdictionary_add_objective_name_method to 288e6f7