Make representation integers exportable and small improvements to backend ppl

[kliem]

In backend_ppl.py we currently manually copy the integers representing the different representation types in representation.py. In this ticket we change this to an import. This is easier to read and less error prone.

In addition we outsource to abstract methods to create a ppl generator resp. constraint system.

CC: @tscrim @yuan-zhou @mkoeppe

Component: geometry

Author: Jonathan Kliem

Branch/Commit: 94bd1ee

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/33678

[kliem]

Branch: public/33678

[kliem]

New commits:

6e8740f	use types directly from representation.py
94bd1ee	outsource creating of ppl generator/constraint system

[kliem]

Commit: 94bd1ee

[tscrim]

Reviewer: Travis Scrimshaw

[tscrim]

comment:3

Personally, I am not a fan of the @staticmethod, but since that is what is done for other conversion functions, let it be so.

[kliem]

comment:4

Thank you.

[vbraun]

Changed branch from public/33678 to 94bd1ee