Inspired by the discussion in #30362, comment:77, we change the Manifold constructor so that once can omit the structure parameter when parameters such as diff_degree or metric_name are given:

        sage: M = Manifold(3, 'M', diff_degree=0); M
        3-dimensional topological manifold M
        sage: M = Manifold(3, 'M', diff_degree=2); M
        3-dimensional differentiable manifold M
        sage: M = Manifold(3, 'M', metric_name='g'); M
        3-dimensional Riemannian manifold M

CC: @egourgoulhon @tscrim @tobiasdiez

Component: manifolds

Author: Matthias Koeppe

Branch/Commit: 9548b50

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/33001