Open subsets of arbitrary subsets of manifolds

[mkoeppe]

Various places in the code assumed that the elements of the _supersets attribute of an open set are open, but this is not a guaranteed invariant.

We fix this by updating all loops over supersets that need to make this assumption to use a new generator function ManifoldSubset.open_supersets.

We also add the method ManifoldSubset.open_subset. It delegates to its base manifold's open_subset method, by passing a new optional parameter supersets to it.

[mkoeppe]

Here's the beginning. To complete it, the remaining open_subset methods will need updating.

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New commits:

​1f6f295	TopologicalSubmanifold.open_subset: New
​58110f2	TopologicalSubmanifold.open_subset: Fix docstring
​d815b63	{DifferentiableSubmanifold,PseudoRiemannianSubmanifold}.open_subset: New
​aac8835	{Topological,Differentiable,PseudoRiemannianSubmanifold}._repr_: Print subsets as subsets
​004c23a	Add/update examples
​ef1614a	Refactor Manifold.open_subset methods through new method _init_open_subset
​265b0e2	TopologicalManifold._init_open_subset: Add docstring
​4e63b30	WIP

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​a93f7ed	TopologicalSubmanifold._init_open_subset: Add docstring
​58b43f7	DifferentiableManifold._init_open_subset: Add docstring
​5623f49	WIP

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​d542412	ManifoldSubset.open_supersets: New generator, use it to fix loops that assume open supersets
​c821a75	{Topological,...}Manifold.open_subset: New optional parameter supersets
​b35eca1	ManifoldSubset.{open_subset,_init_open_subset}: New