induced_subgraph: new method based on a subset of edges

[cecileane]

The package already has a method for an induced_subgraph based on a subset of nodes.

1. I added another method based on a subset of edges.
2. In the documentation, I added an example with an edge subset, next to the example based on a node subset.

While doing so, I noticed that issubset may have a bug, as an induced subgraph is not always a subset of the original graph. I added a sentence in the documentation warning users that issubset is not fully supported yet. The issue is not related to the new function: I added 2 examples, one of which is an induced subgraph from a subset of vertices.

I also noticed that issubset checks the relationship between the underlying graphs only, so I added comments inside the function to note that there's no check on the 2 graphs having consistent vertex labels or edge data.

I'm happy to remove everything about issubset from this PR. But then users following the package documentation may notice that the example induced subgraph is not a subset of its parent graph, and I wanted to be proactive about recognizing the issue.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b769719) 93.37% compared to head (00bc2dd) 93.48%. Report is 5 commits behind head on master.

:exclamation: Current head 00bc2dd differs from pull request most recent head 931bcd1. Consider uploading reports for the commit 931bcd1 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #72 +/- ## ========================================== + Coverage 93.37% 93.48% +0.10% ========================================== Files 7 7 Lines 302 307 +5 ========================================== + Hits 282 287 +5 Misses 20 20 ```

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